For such large amounts, you can't simply calculate the number of bitcoins * price per bitcoin, since the act of selling such large amounts would itself affect the price.

No, we know that there is some fuzzy nebulous number of bitcoin millionaires which may be centered around 25 - more based on how many have coins split across many addresses, and less based on the known fact that many earlier miners didn't store their coins safely (because it was just an interesting crypto prototype they were playing with) and many tens of thousands of coins have been lost. (That Polish exchange running on Amazon without backups took out, IIRC, something like 10k all on its own.)