Earlier quoted context omitted.
The total valuation of those 511 is currently about $11.5 billion. We ourselves don't know the percentage owned by founders. But considering that investors continually complain that YC companies are overpriced, it may be higher than the industry average.
I find it interesting that the current average valuation of a YC company is 22.5 million which is almost identical to what it was 2 years ago when there was 210 companys "the average value of startups we've funded is about $22.4 million" http://ycombinator.com/nums.html
The average valuation of the 268 of those 511 companies that have valuations (by being acquired, raising an equity round, or going out of business, in which case it's zero) is $43 million.