My first shotgun as a kid was a 20 ga. Westernfield 550A as I recall. 28" Imp. Mod. choke barrel. I paid $49 at Wards back in the mid-60's for it. It's a sweet shooter and can shoot all day long with the 12 ga's. guns. I would carry it when we went out hunting quail, pheasant, ducks, rabbits or squirresl and would always bring home my limit along with the adults who were using 12 ga. guns. It's still solid as rock and is in pristene condition. My wife loves to shoot it at clays I throw for her.
Sadly, $150 to $200 is all you're going to get for it these days providing it's in real good condition. The market for shotguns like that is flooded. I doubt if there's any collector's value to it.