In investment terms 'return' normally refers to profit. See http://www.investopedia.com/terms/r/returnoninvestment.asp
So I think you should have ended up with 1240%.
E.g turning $1 to $1.50 is a 50% return and $1 to $2 is 100% so $1 to $13.4 is a 1240% return. Still a spectacular return given the short time period.
Otherwise I agree with your numbers. The cost doubling sounds high at first but YC is by definition very hands on.