That the question. Do we actually have any idea about this ?
Recently have been wondering about this also.
The real answer is, we will never know.
But it would be very interesting to have this information.
If we had information about from all the exchanges, we could have some approximate about the real number of FIAT invested.
A common misconception is marketcap is the amount that has actually gone into crypto, but in reality its far far far less because many people hold their coins and even a 10 million dollar investment can push the market cap up a billion dollars in many cases.
Yes, market cap is something completely different from Real FIAT "number".
But if we think about it, the market-cap of stock-market is the same thing, only in this case one would have far better chance of calculating FIAT "number" .