What is the value of a Browning B80 20 gauge shotgun?

Without any more information, you question is almost impossible to answer with any real degree of accuracy. Value will depend on where you live, how bad you want to sell it, condition, finish, accessories, box, papers, engraving, what choke it is, barrel length, detailed description of all markings, stock wood, etc.. You could be in the 100-1000 USD or more range.