That is an interesting question. I guess one way to approximate a very rough, back-of-the-envelope type of answer would be to look at the average daily dollar volume traded and the total market capitalization. So for instance with the Nasdaq we have approx. $30 trillion in market cap and approx. $400 billion of average daily volume, a factor of 75. So in the best-case situation where each share trades exactly once, it would take approx. 75 trading days for the market to turn over once. However, let's take a more pessimistic assumption (though probably still not pessimistic enough) that shares are selected randomly to trade. We can use the classic ball-and-bins type of probability to estimate how many days it would take until let's say 95% of dollar value had turned over at least once, which produces an estimate of roughly 225 trading days, so let's round it up to 1 trading year (these are very rough estimates). So then assuming that roughly 95% of the Nasdaq market cap has turned over in the past year, then the average market cap over the past year is not a bad ball-park estimate of the cost basis. (I invite any real mathematicians out there to tell me where I went wrong.)