American coins are, in general, more expensive than coins from just about anywhere else. My guess as to the reason for this is that America seems to have a higher proportion of it's population who are coin collectors. That, combined with a large population to start with, creates a lot of demand, which drives the prices up. More and more collectors in the US are turning to world coins, simply because they're cheaper.
Another factor which has increased demand for this particular coin is it's motif and relatively innovative design. America hasn't issued too many coins commemorating the achievements of non-Americans.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis