Do you want to shoot slugs or buckshot? Probably the best bang for your buck will be the New England Firearms Ultra Slug. Should be around $250+/- (I forget how much I paid for mine) and with a scope and good ammo (Lightfield slugs) you can expect to shoot groups that make some rifle shooters jealous.

H&R makes the same gunalso.Infact, they may be made by the same company. Not sure if it's NEF or H&R though.