Yeah, rarity and condition play a huge role in retro game collection, sometimes more than how good the game is. One of the most expensive Atari 2600 carts is a poor attempt at cross promotion by Ralston, yet its price is so high due to the extremely small number of copies around.
Anyway, if you're just into LoM for the music, that's what sites like this exist for:
http://www.tzone.org/~llin/psf/