They are totally worthless. However, you are fortunate in that I am building a recreation of Bannerman's Castle and am in need of some barrels for reinforcing concrete. Send it to me and I will include a memorial plaque on the wall for all to see your generosity.
Actually, there are some bess repros on some of the gun auction sites right now. (One is a cheap POS and should not be confused with a repro Bess) As with anything else, condition and model and faithfulness to the original is what matters most. I think on gunbroker, you can actually check prices over a few months of closed auctions, as well.