Let's assume that they have invested since the beginning of YC with the 250 startups:

   250 * 150 000 = $37 500 000

So far, 20 YC companies have been acquired generating $401 500 000 (data taken from yclist.com). If the investors own an average of 5% of the startups, their current position would have been about 50% loss:

   401 500 000 * .05 = $20 075 000

I don't think this is a bad position to be in since there are still great companies that have not been acquired or exited. The $150k investment may also reduce the failure rate.