The long run stock market returns are just below 8% (https://www.wealthsimple.com/en-ca/learn/average-stock-market-return#:~:text=Longer-term returns,-2018 is about 7.96%.), so I think your 12% is optimistic. Your "low returns" case is I think more accurate.
I'm not sure anything in your post really depends on the numbers, but that extra 4 points is a lot of money in the out years.
My primary concern with cryonics is that by far the most complicated part will be memory restoration. Getting the brain up and working will likely be significantly easier than restoring your memories as they were. I could easily see a future organization deciding to do it "the cheap way", especially since there are no guarantees of memory restoration efforts with most cryonics organizations, and reviving you as a mostly blank slate.
This seems inefficient to me, as it effectively is just an expensive way of adding +1 to the population if your unique insights and experience from the past can't come with you. Plus...we don't know what the future holds. Looking at history, there is no guarantee of positive moral development in our countries. If they revive you as a blank slate, it could be an age of what would to us be moral monstrosities being routinely accepted by society and those values could be imparted to you as they educate and rehabilitate you.
So it could just amount to making an expensive monster. I'm actually very confident in the odds of success for cryonics, but putting my very brain into the hands of a future culture with unknown values worries me.
Of course, the opposite could also be true and they could well be even better than we are, they can restore your memory even better than it is now, and your insights from the past make you a valuable member of that future society.
It's a gamble and I see no way of reasonably calculating the odds on it. I'm extremely interested in cryonics and nearly had a policy in place for it myself at one point, but now I'm not sure I want to roll a dice when I don't even know how many sides it has.
I know there are a few folks signed up for cryonics on LessWrong. At time of writing, I have decided not to do so. This covers part of the research I did while deciding: how expensive is cryonics?
I am in Canada where only one insurance company offers life insurance that will pay out to a cryonics company (I don't know why they care, but that's another question).
A lifetime insurance premium with the company that pays out $180K CAD will cost me $89.45 CAD per month up to 100 years of age.
But researching life insurance taught me lots about compounding interest. I realized that due to compounding interest, I can do better than the above by getting 20 year term insurance. If I do that, I will pay $16.60 per month for the first 20 years, $74.50 for the second 20, and $368.93 if necessary after that.
Assuming even low stock market performance of +7%/year, if I just invest the difference in lifetime vs 20 year term premium costs, compounding interest on that difference will give me the $180K after 45 years, plus extra cash the longer I live. At 12% growth per year I'd have the $180K after 32 years and $20M by the age of 100.
This taught me that regular savings deposits while I'm still young are a great idea. It also persuaded me that thinking about the cost of cryonics as a $20 per month subscription is not a great idea. Cryonics is a commitment to a five- or six-figure payout in your will that could have been inherited by another individual or organization, and that '$20 a month subscription' is just insurance in case you die before you've saved the requisite funds.
A five- or six- figure sum + membership fees + insurance when you're young in exchange for even a low probability chance of dodging death still sounds like a good deal to me, and I suggest advocates be more deliberate about framing the cost of cryonics in these terms.
(An aside: There are cryonics advocates who've written about why it isn't cheap, and you can find a different take to mine from Rudi Hoffman here. He doesn't address the effects of compounding interest, and I think they are a strong argument against his claim that 'the best life insurance for anyone to have is the one that is in force when you “die.”')
If you're living in the US, you can find a company responsible for standby, stabilization, and transport to your Cryonics provider that accepts payment from your life insurance and will serve you anywhere within the country.
Outside of the US, this may not be the case. The closest provider to me requests a $7,000 prepayment to arrange the above, and if I move outside their area of service, I'd have to find another provider that might not exist.
This is an uncomfortably large upfront cost for me, and the potential complications of moving are even more offputting. That said, it is an order of magnitude cheaper than CI's US partner Suspended Animation at time of writing.
This was simple: a lifetime membership at CI is approx. ~$1,600 CAD.
I'm assuming 12% YoY stock market growth, a long life, and no price rises. But I'm still hypothetically insuring myself at $180K in case those prices rises happen!
Upfront fees: ~$8,600 CAD
Insurance: $14,712 CAD over 32 years
From estate after death to cover procedure: ~$35,900 CAD
Total: $59,212 CAD (plus any future price rises)
Depends on the probability of success! That's a separate post.