Every few months or so, I get a link to an out of print RPG book on Amazon with a ridiculous price. The third-party seller usually wants something like a couple grand or so. This is usually caused by the pricing algorithm getting bad info and ballooning the price up in a vacuum.
Half-Price Books has gotten a bit smarter with their pricing of OOP RPG stuff. Gone are the days where you could find a pristine copy of Keep on the Borderlands for a couple bucks. Hard-to-find stuff is now usually at least cover and, if you've got a knowledgeable member of the staff who pays attention to eBay, more.
I was in a Half-Price today and came across some 7th Sea books in their collectors case. Most of them were cover or more. They had a copy of a very rare 7th Sea book: Swordsman Guild. The price reflected the rarity - $200.
The Swordsman Guild tied for the most expensive book they had in the store. The other $200 book? A copy of Mein Kampf from the 1930's in the original German, complete with Nazi Germany symbol stamped on the front.