If you are in the diamonds business, you can easily subscribe with Rappaport, but if you are not, it is a copyrighted list that is ONLY available to those in the trade.
Technically it is NOT for consumer use, as it can be complex, and even the price stated for the carat, color clarity, isn't the price to be paid. It can be more or less, depending on how far from ideal the cut is, or sometimes higher if it is a branded stone with a premium...
Try using PriceScope's Price Stats to determine what average and low price points can be. Better yet, try typing the specs of the stones into the PriceScope search engine at the home page, and you can get a good average...