YC invests more like $17k on average, since most startups are 2 people. So your base is wrong there.
You also have to assume dilution since almost everyone raises money. So they might have started with 6%, but they probably have more like 3% in a mature company (assuming 50% dilution for no particular reason).
So if you wanted to estimate returns at this point you could say (3% * 4.7 billion) / (212 * 17k) which is 39x (or 3900%). Not half bad!
You also have to account for the other services and facilities that YC provides. I believe pg has mentioned previously that the total cost per startup for YC is about $40k. That would be about a 16x return. Still pretty amazing.
You also have to assume dilution since almost everyone raises money. So they might have started with 6%, but they probably have more like 3% in a mature company (assuming 50% dilution for no particular reason).
So if you wanted to estimate returns at this point you could say (3% * 4.7 billion) / (212 * 17k) which is 39x (or 3900%). Not half bad!