I know rarity is a huge factor when it comes to games ( sold an NES game called Color a Dinosaur for $120 and it's just rare even though it sucks ) but Gamecube games are hardly rare. I get a Wind Waker or a Melee or whatever in one of the stores at least once a week and yet those are like $70+. There are millions of copies of those.
I blame the Ebay sellers who selling it for like double that and people just....pay it.