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]
[0] http://motherboard.vice.com/blog/bitcoin-mints-its-first-bil... [1] http://www.forbes.com/billionaires/list/