There are lots of "rare" games. Unfortunately, they're not worth much. There are also "popular" games, which are typically worth more than simply "rare" games because they have name recognition. Sierra and LucasArts made many popular/collectible PC games, but they weren't "rare" by any stretch of the imagination.

There are only a few examples of games that were both popular and rare. The first Akalabeth/Ultima release was rare and super valuable.

EDIT: And of course magnets affect floppy disks ...