Personally, I'd pay $20 for DWARF FORTRESS.
If I can have fun with a game and the price fits that fun, I don't care if it's on the PS3, DS, or Dos 3.1.
Heck, I went and bought River City Ransom for $15 at a Funcoland around ten years ago just because that game is an amazing brawler/RPG combo (with co-op!). God I love that game... totally worth it to plug it into my NES. Old technology? Bah. A unique, fun, experience.
... Oh, and uh... I'd pay $50 bucks for the Monkey Island games if they ever rereleased...