When you take your diamond into a Pawn shop, they generally don't have a gemologist on staff who can evaluate your diamond. The process a pawn shop uses is often as simple as testing it with a diamond tester to confirm it is real, measure the size of the diamond and offer a price based on a quick visual inspection.
Needless to say, this amount will always be less than you paid. The average price paid for a 1ct. diamond ring from a pawn shop would be about $150 - $200. Remember, they are not buying the diamond, they are using it as collateral against a loan... you are actually supposed to get it back when you repay the loan.