I don't know how the pawn shops are in your area, but most will move off the asking price if you show cash, and are serious about buying.
Just a conservative guess, but I would bet the shop in in the gun for something between 100-150 dollars. Generally speaking, most pawn shops ask about 40-45% more than they have in an item. They usually take the item in a a price far less than a "book" value, taking advantage of a person that normally desperately needs the money. This may seem hard, but they are there to loan money and make a profit.
Make an offer of 175 cash and see if they will dance with you, if not, let them know you want the gun, but are not willing to pay the asking price, so what is your absolute bottom dollar. I'd bet he comes off and cuts you a deal, particularly if he has had the gun for a while.