I bought Castlevania SOTN from Play-N-Trade for $25. Got Xenogears from eBay for about the same price. And I see Crash and Spyro games at places like thrift shops and Goodwill all the time, looks like they trend about $10-$20 on eBay.
Damn...I wish I had thrift shops like that around me. There are a few, I guess I'll have to go check them out.
I guess because of what I am looking for specifically in a game like Xenogears or Castlevania SotN, they are much higher. These games are "cream of the crop" titles. So if I buy them, they have to be near-flawless, original black label versions. Those are pretty expensive.
On a side note, my package with seven black label original PlayStation Final Fantasy games arrived yesterday. Oh man...the ONLY thing wrong with the entire set is the bottom right corner of the FFVII booklet is slightly bent. Not dog-eared, just bent downward. It's a beautiful set that any discerning gamer would be proud to own.