The PC collecting market is so silly weird. You know something is amiss when junk like Duke Nukem: Nuclear Winter moves for $50/disc only while gold like Duke Caribbean is lucky to hit that much complete.
Lots of neat little odds and ends that cost a pretty penny, though. I'd love to get my dick beaters on a complete copy of One Unit Whole Blood sometime. That's like, the ultimate Blood pack.
It's always kind of bummed me out that Looking Glass' best games get as ignored by collectors as they did by consumers back when they came out. You can pick up new/sealed copies of Terra Nova for as little as $10; it was easily their biggest flop (and a major reason why the studio died) and yet it's one of my favorite games from them. Incredibly innovative and the controls still work great.