If there is a printed guide to the sales prices of some set of stuff, you can bet that the regional or national pawn chain store has a copy under the counter or in the back. It is worth money to them to know this information. Local pawns may or may not be as well informed. Something that has been sitting there for a long time is more amenable to bargaining. Every object for sale carries a price tag.
Local pawns are cyclical.
Just before payday when lots of money is going out, inventory coming in, is the best time to bargain with these shops. There is never a good time to bargain with a national chain pawn, as the larger number of stores and wider dispersion of shops allows them to average such cycles and move excess inventory to other stores where it's selling. Knowing and being recognized by the proprietor of the shop may be a help. Maybe not, too. Depends on the store and the people working there.