Let's see, if I remember well they invested about 80 000$ in reddit (there was a second investment from YC if I'm not mistaken). So from this, they maybe took 10-15%. Reddit sold for about 12 million dollars, so that would mean about 1.2-1.8 million: 2-3 years of yc investment.

Of course those numbers are pure speculation and it wouldn't suprise me if I'm completely mistaken...