Apparently Satoshi has around 1 million BTC [0] which is staggering plugging your numbers. That'd put him at $500 billion which is insane. Since you said 20 million BTC (less than the total there will ever be), we're talking within about 20 years right? It would be incredibly interesting to see what happens if somebody ends up with a theoretical worth exceeding the top 10 wealthiest people in the world, but I wonder whether the mystery of who he is could actually stop that happening. Also only 4 individuals are worth more than $50B so that's pretty incredible too. [1]
Inflation adjusted, that would put him in the range of Rockefeller, Carnegie, Vanderbilt, Mellon, etc. If (and it's a big if) bitcoin was worth as much as the world gold market, it isn't disruptive like an internet startup, it's disruptive like Marxism. We could argue about the insanity suggesting that bitcoin will reach that sort of value, but if it somehow did, then $500 billion isn't totally out of line.
I haven't really researched either character, but Gavin seems to claim/imply he isn't Satoshi.
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This is quoted from the defunct Bruce Wagner Bitcoin podcast:
"Bruce Wagner : When was the last time you chatted to satoshi <laugh>
Gavin Andresen: Um... I haven't had email from satoshi in a couple months actually. The last email I sent him I actually told him I was going to talk at the CIA. So it's possible , that.... that may have um had something to with his deciding
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[0] http://motherboard.vice.com/blog/bitcoin-mints-its-first-bil... [1] http://www.forbes.com/billionaires/list/