Their most successful sale is still Reddit, yes? Assuming moderate dilution, I imagine payout per founder in the $1mm-$2mm range. Assuming the average ycombinator is capable of securing a 6-figure job, their greatest success only accounts for 2 years living wages of 10 failures.
Forget the multi-million dollar company valuations: almost everyone on this board would have a multi-million dollar valuation if they were willing (or legally able) to sell ownership over all their future cash flows.
Please note I have the greatest respect for entrepreneurs and what they do, I'm just making the point that what they do is not economically motivated in general. I'm also not making a comment on whether ycombinator itself does well - their investments are much smaller than forgone wages.
I'd take that wager at a 100:1 payout in a heartbeat!
My guess is that it takes 5-7 years for payouts to start happening from startups, so allowing for that much time, I have no doubt that the total payout will be much greater than would be from regular salaries. All you need are a few to hit it out of the park, and you've covered everyone.
Of course, we're talking about aggregates here; I could certainly imagine that for some fraction of the participants, it might be a net lose monetarily.