I've been looking at a few bulk lots of NES games and have seen Maniac Mansion listed on multiple occasions as rare. True? I have a whole pile of them.
I have read that too, but I (like you) have had many copies of the game. I still haven't seen them fetch more than a couple dollars for a loose cart. There is an auction on eBay right now for a cmplete game and it will fetch an okay amount. Probably nothing spectacular though.