You could iterate through each block and multiply the reward by the price at that moment. That would give you all of the value that was created rather than exchanged in a balanced exchange. Someone either sold that at that rate, or held them (opportunity cost paid). You could estimate it fairly easily by breaking the price up in to monthly averages (not weighted by volume), and using 100k btc for each 25 btc month, and 200k btc for each 50 btc month.