pg hit the home run on this one
And direct investment costs for 208 startups is <$500k. Even if you add in all the other costs of running the program, YC is pretty f-ing successful.
Then you start thinking about the other startups yet to exit (ie, Loopt, Dropbox) and you see the YC team is truly kicking ass.
Still, YC has still returned (and potentially as much as quadrupled) direct costs already, without a number of companies still to exit.