It'd be hard to figure out. A lot of hedge funds run on mark-to-market accounting, which basically taxes the unrealized gains as gains every year, as opposed to keeping track of the cost basis when bought and taxed upon selling. There are quarterly reports made to the SEC though for firms of a certain size, or holding a certain percentage of one stock, typically 10% of a company involves additional paperwork and regulation.

Also, there's many times where the cost basis is stepped up, for example when someone dies and passes on assets to their beneficiaries. In this case the cost basis changes from what the dead person paid for it to what the beneficiary gets it for upon inheriting it.

Overall, I don't think this is a useful measure of anything though. What you might be interested in is the volume weighted average price, which gives an idea on how much people have paid on a stock in a recent window, based on the volume of trades and the price of those trades, reflecting what the average buyer's cost basis could be around.