How much for used accordions?

It's all over the map. Late model used accordions from top makers sell for thousands of dollars. Old, worn accordions with condition issues can be hard to give away. The standard advice is to deal with a reputable dealer, but bargain hunting can yield good results. One thing to check out is whether the seller knows anything about accordions. A player should be able to tell you how many sets of reeds there are, when it was last tuned or serviced, and if there are any issues. A price of $400 would be a steal for any genuinely good, playable accordion, but it might also get you a musty out of tune wheezebag. You basically either have to find a player who can help find an appropriate instrument or take your chances and consider it as tuition. Good Luck!