The mean YC startup about a year ago [1] $22.4 million, let us assume 3 founds on average and 3 years of work, and that they hold 1/3 of the value, and you get an average return well north of 500k a year per founder.
And most of that money doesn't actually go to the founders - YC owns a chunk of them, and so do other investors (as PG says, 94% of them get funding from elsewhere too).
[1]http://ycombinator.com/nums.html