The value may depend on where you live as well as the condition of the gun. Up here in Canada they aren't very popular and sell for about the same or a bit more than a single shot ($100 to $150). I had an older model 20ga (model 185D ) that had the Mossberg D-tachable chokes ( early version of choke tubes). The "K" models were more common, these had Mossbergs C-let choke (adjustable poly choke). I beleive there were also some fixed choke verions as well. What choke set up does yours have, and is it 2-3/4" or 3" chamber?
Jim