Earlier quoted context omitted.
Probably around 0.06 * 10,000,000,000 Assuming no subsequent investments, dilution, selling of shares, etc.
So 600M is the upper bound. Drew Houston owns roughly 25% of the company now, and he probably owned ~75% at when YC invested, for a dilution rate of 3. I'd say YC investment probably has dilution rate of 10, for the real amount of 60M. Not bad, but not Whatsapp scale either.
Helping other people realise their dreams and only making 8/9 figures in the bargain. Can't imagine why they would bother.