Mike, I used to work for a numismatist and jeweler, & actually have a NGI certification under my belt.

To find out what the stone is worth, you need to know what the size and rating is for the stone, then ask to have it compared against a recently published Rappaport sheet which is a very commonly used market pricing index for diamonds. It is used for sales, and I think it would be fair for you to expect a price in the range of 15-20% less than what the stone is going for on Rappaport.

The finding, or ring setting, is only worth it's melt down weight + quality (14k, 18k, etc...) during resale.

edit: Warning, Pawn shop owners don't like customers who know what they are talking about. I'd suggest going to a jeweler that buys and sells, ask them to Rappaport the stone for you, and you'll come across as knowing WTF you are talking about and get better treatment.