YC's stake is common stock. It was presumably somewhere between 2-7%, like is advertised for current YC companies, back then. I assume it got diluted a lot since then, as Dropbox has had multiple rounds of financing.
Dropbox is worth maybe $5b. I think YC's stake comes out to 1-2% after the rounds. So, off by an order of magnitude or so. Just based on published info about the distribution of YC companies and their value, it's probably more like $200mm total fund value. Given that money-in was say 300 x $20k, and maybe triple that to account for salaries and other overhead, say $20mm, it's a 10x return so far.
Which is pretty baller for a VC fund.
On the other hand, I suspect PG etc. could have created a single startup of their own worth >$300m x 2 (to account for dilution) and probably exited it, in the same amount of time.
I wonder. YC's initial stake is usually <10%, right? Dropbox got $1.2MM right after YCombinator, which to me suggests a high rate of dilution. So let's say YCombinator owns 5% of dropbox. Dropbox is probably worth slightly less than $6bn according to pg's comment that Dropbox + AirBnB are worth about $7.5b. That would value YC's stake in Dropbox at about $300MM.
Of course, this is just a guess-I have no idea how much Dropbox was diluted in their early funding rounds. Even so, Ycombinator's stake probably couldn't be worth more than $600MM if we assume Dropbox is worth $6bn.
YC could probably sell their stake in Dropbox alone for more than that.