Not sure whats a decent price. I do see gander mountain has a rem 20ga 3" cantilever 18" for 269.oo/. # 401216
If you have a good LGS see what they can do for a reference on price.
Heres one for $211.80 -http://www.rrarms.com/catalog.php?prod=R7599 That would be R&R arms 218-759-1866 item # is R7599