When you see the prices you probably see a certified grade with them? Look at EBAY. people send them away to be graded. A perfect coin is a 70. What one buys from the mint will not always be a 70. Generally early strikes are better. A 70 grade is much rarer than a 69 which should account for the difference in price. Ungraded for that specific coin will not sell for the same price as a graded coin. I forget how much it costs to grade. Like $25-35 i believe. CGC is a good site. They grade comics, coins, magazines.
Does kind of make me think about it though. If you get really lucky and get early strikes on your proofs, you can make a mint (uh, no pun intended), but in order to tell, unless you're an expert in such things, you essentially have to pay almost half it's value (if it's a standard proof) to determine if it's a better one. Sigh.
Still, it's a beautiful coin. Love the proofs. I have the 75th anniversary set where they did the normal and 'reverse' proof. It's freaking astounding. I'm thinking that set would probably be worth having it graded.