I think you might be missing something more obvious here: tech has a huge amount of slack when it comes to money.  If I were running a tech event of similar size to what you described, I wouldn't bother charging, because it would be a waste of my time.  When you make half a million dollars a year, funding something like that yourself basically comes out of your fun budget; you don't really even think twice about it.

Yoga and new age groups though?  Not nearly as flush with cash.

Money and spirituality are a tricky topic.

I'm currently president of the board at my zen center, so I see and think a lot about our finances. We run pretty lean. We ask members to pay dues to support the center, and we hold events where we ask people to pay. 90% of this money though is to cover rent, insurance, food during retreats, and various incidentals. Our teacher gets a very modest stipend, and he couldn't survive on it alone.

We sometimes debate things like how to deal with people who can't afford to pay our recommended dues and fees for events. In every case so far we've said "sure, pay what you can, these are just guidelines based on what it takes to operate, but we won't turn you away from the dharma because you don't have money".

I know that outside zen (and sometimes even within zen!) you find people who try to make a living by doing spiritual work. I think there's nothing necessarily wrong with this: our economic system is structured such that money is how we allocate resources, and even if you want to be a mendicant, rather than begging directly for food, shelter, etc. we ask people to beg for money so they can buy their own food, shelter, etc. because that's how our system works. But of course once you bring money in, because it's fungible, people can try to enrich themselves, so more discipline is required.

So the optimal amount of money to be asked of in spiritual spaces is not 0, but it's also not very high. Sounds like the vibes of this place were off, and that's a shame.

One parting thought: some time ago I saw a tweet that I can no longer find that address the phenomenon you're noticing rather succinctly. I think it went something like this meme:

(I don't know if you're in America but this is a very American thing to turn stuff into jobs. I think it spills over to other places because we're really good at exporting our culture.)

Even if I am willing to teaching for free, the room costs money; so I would consider it fair if the participants pay for the room (and possibly other material costs, such as food, printed materials, maybe also my travel costs). I may not want to make profit from my spiritual activity, but I do not want to make loss either.

If there is a huge demand for my teachings, I should also consider the fact that if I keep my daily job to pay my bills, and only teach during evenings or weekends (most likely not all evening and all weekends, I also need to take a break, and maybe have duties beyond my job and teaching), I can only teach maybe 10% or 20% as much as if I instead quit my daily job, and charged more money to pay my bills. So even if my motivation is purely altruistic, getting paid might be the right choice, if there are enough people willing to pay the increased price to keep me doing it as a full-time job. (The alternative is that most of those people are sent home: sorry, the room is full. And the room is also full the next day, and the day after that.)

So this is an argument for taking money. It does not say how specifically the money should be taken. Donations (perhaps with a suggestion like "the recommended amount to cover our material costs is X") might work, though I would suggest asking for 50% more, to compensate for those who won't donate. Or you might get a millionaire donor (you might want to provide them info such as "paying X would cover our expenses for one year"), in which case you do not need to change the participants.

There is also a valid argument against charging money, which goes like this: There is a tradeoff between optimizing for money and optimizing for the spiritual message. If you lean towards money too much, it will come at an expense of the quality of your teaching. As the Bible says: "You cannot simultaneously optimize for two different utility functions. Maximizing one of them requires sacrificing the other, and vice versa." (Luke 16:13)

Sounds like the solution is to simply not maximize for money; charge only as much as you must, and otherwise optimize for the message. That is the intuition behind the idea of "just price". But this is easier said than done, because we are running on corrupted hardware. What is the just price of my lecture?

In case of covering the financial expenses, it seems easy: just collect the bills, and then collect enough money to pay them. But even here, there would be a temptation to choose a more convenient room, a more tasty food, a more convenient form of travel. To print my spiritual teachings in a hardcover color book, instead of some cheap black and white printout (and a freely downloadable PDF as an alternative). Even if I resist the temptation to put a single cent in my pocket, my brain will find a way to extract some other value, along with a convenient excuse why doing so is the right way to run my business... ahem, a non-profit spiritual mission. Until one day I find myself traveling all around the world by plane, living in five-star hotels, eating caviar, and wearing a huge gold watch (hey, I need to know the exact time in order to deliver my spiritual message optimally, and these expensive watches are very precise and reliable, you should really try them). But hey, no money in my pocket, I just live an absurdly luxurious life with all expenses paid by the believers; and I got there step by step.

How much money do I need to pay my bills, that also depends on how large my bills are. Should I take as much as I made at my recent job; perhaps adjusted for inflation? Or should I say "hey, hypothetically I could get a job in a Silicon Valley, or maybe become a CEO of some successful startup, so it is only fair to get a compensation for giving up that". Or I may simply focus on my living expenses... but they keep growing without limits.

These may be silly examples, but the idea is that there is a slippery slope from here to there, and without a simple rule where to stop, I will be tempted to gradually push further and further, always telling myself that I am a virtuous person, because I am not going as far as I possibly could.

Also, what about advertisement? At the minimum, it would make sense to announce my lectures somehow. Otherwise, how would people know when and where to go? At the far end, there are billboards with my face all over the town (many towns actually), and I am playing an expensive zero-sum game against my fellow billionaire evangelists.

These are the concerns that make "do not take any money, ever" and "only take voluntary donations" seem like the only Schelling points. Even these are not really safe. For example, I may recruit dozens of volunteers, working for free (idealistic students are the best), collecting donations for me all over the town. And if I already have crowds of enthusiastic true believers, I might be tempted to also use some of them sexually.

So... what is the answer here?

I guess, it is "with great power comes great responsibility", and realizing that having an attractive spiritual message is a great power. There are no best answers. Try to resist as long as you can.

But from the perspective of a "customer", it is perfectly valid to decide that certain person or organization is already too far on the slippery slope of temptation, and to refuse to engage with them.

There's a question of how the resources that result in an event get sourced. 

The more resources people in a community have, the easier it is for them to run events that are free for the participants. The tech community has plenty of money and therefore many tech events are free.

A teacher of practical skills ("How to Change a Light Bulb 101") may reasonably expect to be paid for their services. A purveyor of values (politics, religion, an academic field that claims to be purely descriptive but really isn't, "good karma", moralistic art, rationalist philosophy, etc.) should not.

If you categorically don't pay people who are a purveyor of values, then you are declaring that you want that nobody is a purveyor of values as their full-time job. 

If someone runs a libertarian think-tank, to spread libertarian values, they are usually funded through donations because someone considers it valuable to spread libertarian values instead of being paid by the audience of the think tank. That's because there are libertarians who have money to spare and who value spreading libertarian values enough to donate to it. 

It's like whether or not a website is free or paid. Plenty of websites are free because they like an audience that watches the ads on the website. Many events that are about spreading values are free because they exist for making the audience believe those values. 

To the extent that there's value to having events by people who don't organize them besides their full-time job but are more focused on creating those events and there's no grant/donation money to run them, taking money for them allows the events to exist. I think there could be a lot of value generated by having more people organize valuable events and take money for them.

Checking people's tickets during the events still sounds strange and suggests that there was in the event you are speaking about too much focus on money. It makes more sense to charge a price for admission, check at the entrance, and then leave the topic of money out of the actual event. Or alternatively, do it the way the church does and have no entrance fee and ask for donations during the event.

In Buddhist ideology, the reason to pick one set of values over another is to find an end to suffering. The Buddha claimed that certain values tended to lead towards the end of suffering and other values tended to lead in the opposite direction. He recommended that people check this claim for themselves.

In this way values are seen as instrumental rather than fundamental in Buddhism -- that is, Buddhists pick values on the basis of the consequences of holding those values, rather than any fundamental rightness of the values themselves.

Now you may say that the "end of suffering" is itself a value; that there is nothing special about the end of suffering except to one who happens to value it. If you take this perspective then you're essentially saying: there is nothing objectively worthwhile in life, only things that certain people happen to value. But if this was true then you'd expect to be able to go through life and see that each seemingly-worthwhile thing is not intrinsically worthwhile, but only worthwhile from a certain parochial perspective. Is that really the case?