However, if your old harmonica is shaped like a zeppelin, an automobile, a fish, a boomerang or a rooster, or if it features art deco style paintwork, or has bells, pipes or horns attached to it, then you might possibly be holding something of value. The chances are, if your harmonica just looks like a harmonica, then it probably isn't worth much at all. Even though many of them were quickly consigned to the garbage by parents with headaches, there are still a lot of them out there in boxes of unwanted toys, belongings of deceased relatives, junk stores and so on. Harmonicas were made literally by the thousand and even sometimes by the million (Hohner, just one of the many companies to have manufactured them, produced their one billionth harmonica more than a quarter century ago). Whilst old guitars are often considered to be "vintage", old harmonicas are generally considered to be merely something that has been in someone else's mouth. If you think you've found a valuable old treasure in the attic, it's likely that you are going to be disappointed.
I know that probably doesn't help very much, but unlike used cars and collectible pottery, there is no "blue book" of used harmonica prices. This is probably the question I get asked the most by email and the best answer I can give is that like anything else, an old harmonica is worth whatever someone is willing to pay you for it.