If the books are in mint condition, you can get pretty decent money for them, but the Man of Many Faces volume I mentioned seems to be the most collectible. There was one used copy for sale for $20, but the rest were going in the $35 to $200 range, with new copies starting at $75 minimum. It's kind of crazy.

Luckily, I don't care much about condition, so I just paid a few dollars each on the ones I ordered. In some cases, the shipping was more than what I paid for the book.