Mo - I bought a concertina last month and haven't regretted it for a second.

As for the "cheap ones", mine cost £500 - which took a bit of saving up - but it was well worth it. As I understand it, anything you can get for less than about £300 is quite likely to disappoint. (This refers to English concertinas, a.k.a. the kind where each key plays one note and one note only. The confusingly-named Anglo variety, which do different things depending on whether you're pulling or pushing, come a bit cheaper.)