As @faul_sname notes, the $8T number (or $9.5T from on source cited in that wikipedia article) isn't plausible. At least, not without some very generous definitions of "cybercrime," "is," "costing," and "trillions."

By which I mean: if you squint really hard, and count all the money and time everyone everywhere is spending on all (digital and non-digital) cybercrime prevention and countermeasures, and try to estimate all the extra things people could do to generate value if they didn't have to worry about cybercrime, then sure, maybe you could get numbers up to a few trillion. 

But that's a bit like saying the cost of other crime includes all spending on the criminal and civil justice system, all spending on private security and surveillance by individuals and businesses, the entire salary of every cashier (since they wouldn't be needed if people would just count up their own purchases and leave payment), and every time someone doesn't do something because they don't want to go out wandering by themselves at 3am. Not actually a useful metric for deciding where it's worthwhile to increase or decrease resource allocations or to make regulatory decisions.

I don't know the answer to how much cybercrime is really costing, but I think your economic analysis is not accurately tracking "what GDP means".

Arms length financial transactions of "money points for services or goods" operates on the basis of scarcity, monopoly pricing power, and other power concerns that are locally legible inside of bilateral exchanges between reasonable agents.

GDP does not track the "reserve price" of consumers of computational services, where conditional on a computing service hypothetically being monopolistically priced, the person would hypothetically pay a LOT for that service.

Various surveys and a bit of logic suggest that people would hypothetically pay thousands or in many cases even tens of thousands of dollars for access to the internet even though the real cost is much much less.

By contrast, GDP just measures the "true scarcity... and lawful evil induced scarcity" part of the economy (mushed together and swirled around, so the DMCA makes hacking printer ink cartridges full of producer-added malware illegal, rather than subsidizing such heroic hacking work, as would occur under benevolent governance, and so on).

Linus Torvalds is probably owed a "debt of gratitude", by Earth, on the order of many billions, and possibly trillions, but he gave away Linux and has never been paid anything like that amount, and so the value he created and gave away does not show up in GDP. (Not just him, there's a whole constellation of rarely sung heroes and moderately happy dudes who were part of a hobbyist ecosystem that created the modern digital world between 1970 and 2010 and gave it away for free).

On a deeper level, the inability to measure or encourage the "post-scarcity" or "public goods" part of the human "economy" (if you can even call it an "economy" when it doesn't run on bilateral arms-length self-interested deals) is part of why such goods are underproduced by default, in general, and have been underproduced for all of human history.

Within this frame, it seems very plausible that the computational consumer surplus that cybercriminals attack is worth huge amounts of money to protect, even though it was acquired very cheaply from people like Linus.

Presumably humans are not yet in "private scarcity-based equilibrium" with the economics of computation processes?

In the long run it might be reasonable to expect the "a la carte computer security situation" (where every technical system becomes a game of whack-a-mole fighting many very specific ways to ruin everything in the computational commons) to devolve until most uses of most computer processes have almost no consumer surplus, because the costs of paying for a la carte help with computer security almost perfectly balances against the consumer surplus from using "essentially free compute".

This would not happen if good computer security practices arise that can somehow preserve the existing (and probably massive) consumer surplus around computers such that "using the internet and computers in general in a safe way is very cheap because computer security itself is easy to get right and spread around as a public good with nearly no marginal cost".

Like... hypothetically the government could make baseline "secure and super valuable" computing systems.

But it doesn't.

A private ad-based surveillance and propaganda corporation "solved search and created lots of billionaires" NOT the library of congress.

The NSA tries to make sure that most consumer hardware and software is insecure so that the <0.5% of consumer buyers that happen to be mobsters or terrorists can be spied on, rather than putting out open source defensive software for everyone.

People like Aaron Swartz and Moxie did, mostly for free, the thigns that a benevolent government would do if a benevolent government existed.

But no actively benevolent governments exist.

In Anathem, Neil Stephenson (who is very smart, in a very fun way) posits a giant science inquisition that prevents technological advancement (leading to AGI or nukes or bioweapons or what have you) and lets humanity "experience the current tech scale" for thousands of years with instabilities factored out and only locally stable cultural loops retained...

...in that world it is just taken for granted that 99.999% of the internet is full of auto-generated lies called "bogons" that are put out by computer security companies so as to force consumers to pay monthly subscriptions for expensive bogon filtering software that make their handheld jeejaws only really good for talking with close personal friends or business associates. It is just normal to them, for the internet to exist and be worthless, like it is normal to us for lies in ads and on the news to be the default.

Anathem's future contains no wikipedia, because wikipedia is like linux: insanely valuable, yet not scarce, with very few dollars directed to it in ways that ensures (1) it isn't hacked from the outside and (2) the leadership doesn't ruin it for personal or ideological profit from the inside.

Anathem offers us a bleak "impossible possible future" but not the bleakest.

Things probably won't happen that way because that exact way of stabilizing human civilization is unlikely, but Anathem honestly grapples with the broader issue where information services are (1) insanely valuable and (2) also nearly impossible for the market to properly price.

This is not an answer, but I register a guess: the number relies on claims about piracy, which is to say illegal downloads of music, movies, videogames, and so on. The problem is that the conventional numbers for this are utter bunk, because the way it gets calculated by default is they take the number of downloads, multiply it by the retail price, and call that the cost.

This would be how they get the cost of cybercrime to significantly exceed the value of the software industry: they can do something like take the whole value of the cybersecurity industry, better-measured losses like from finance and crypto, and then add bunk numbers for piracy losses from the entertainment industry on top of it.