I remember when Earthbound was $30-40 cart only and ~$100 complete five years ago. Price has risen quite a bit.
It's a decent game but it's extremely overrated. The game is just a Dragon Quest title with a real world setting. It's mainly such a highly rated game due to word of mouth, mostly from people who haven't even played the game. Being a character on Super Smash Bros, Melee, and Brawl gave the game more fans.
Good game + amazing packaging + guaranteed to never be re-released + overrated Nintendo property = fairly expensive collectors item