eldequello has given you excellent advice. Don't forget to also check the local classified ads in the "sporting goods" or "firearms" section. The buys are there if you do a little research and go check out the guns in person.
I once bought a Mossberg Model 500 in 20 ga with 3 barrels for $75.00 and it was in EXCELLENT shape. It had a vent rib with the screw in choke system (only had 2 chokes) a 18 inch "home defense" barrel and a 22" rifled barrel. I practically stole it! You never know, used guns can be a real buy.