How much is a 100 year old accordion worth?

Probably not as much as you think. Chances are it was not regularly maintained, so its playing value is probably non-existent. If it is a beautiful piece it might be worth something to somebody. it really depends on the what kind of shape it is in. Tuning an Accordion is a difficult process which can often cost 1,000$ for an overhall, and the chances are the accordion would be worth less than if tuning was invested in the accordion. If it is an average 100 year old accordion you might get 200$ for it on ebay.