One possible stab towards a response is to observe that the cost for most popular sites is dominated by headcount costs, and TPB didn't have a large headcount.
Looking at the budget for Wikipedia, I can not see a headcount cost. There is hardware and bandwidth costs, which should be very similar to the running costs TPB had when the trio ran it.
The question here is what the running costs of TPB is. My point is that those should mirror quite nicely the running costs of Wikipedia, since neither has "headcount cost" attached to each visitor. Both are mostly just serving static text on a website, and neither are paying royalty or click-throughs to third-parties based on visitor count.
In revenue, Wikipedia has donations and TPB has ads. Asking which revenue is bigger is very different from calculating costs. TPB might earn more revenue, but to figure that out one has to calculate the competitive price of advertise space for infamous websites. My guess is that the competitive price is quite low, sine few companies are willing to send untraceable money in order to advertise on a website that many companies and governments consider criminal.
So for revenue, I would look into what botnet spam and hijacked webservers brings in per eyeball. Multiply that with the visitor count of TPB, minus the running cost, and you should get a rather realistic number for the sites total income.
There's a quote from 2006 saying that their monthly hosting costs where 15k SEK (2k USD). But also, one of the owners of the hosting company was also the one running the tpb site.
They were not even at the height of their popularity in 2006. And that (very new) hosting company was raided and all 190 of their machines were confiscated, even though only 11 were being used by TPB.
They scaled up a lot from that point too, so that is not an accurate picture of their costs at all.