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.