boating question how much is boat worth?

my dad has a 1988 sweet water pontoon boat with a hard top 28 horsepower engine with a trailer obviously 20 years old tires are shot on trailer from lack of use so don't know what to ask for it so does anybody know what it is worth not used hard they only ran it up and down a river to a set of lakes that connect to the river if that makes any difference if i can sell it it means free docking for me in exchange for his use of my new boat