Actually, Blue Nile has competitive pricing.
Retailer normally buy diamonds according to a Matrix pricing that is printed by a company called Rapaport. It will tell you what the current pricing according to Carat, Color, and Clarity. It is printed weekly.
Retailer will normally buy diamonds with a negotiated percentage off Rapaport. For GIA diamonds, most retailer will buy them 20% - 35% off Rapaport. 25% off is called "25 back" or "25 off rap" in industry terms.
If you want a better lab and cert than GIA, get a AGS cert diamond. AGS includes cut quality in there cert. AGS normally goes for 10% more than GIA.
GIA lab has gotten some bad press in the pass few years. There was a huge lawsuit against them for improperly grading some of their diamonds.
For other labs and certificates, you can get up to 60% off rap. But other labs are frequently off on color rating and more. They can be up to 2-3 color grades off.
Keep in mind, these prices are for wholesale and they buy diamonds frequently and in quantity. This will give you a rough idea what kind of profit they are making during your negotiating.
Diamond prices have been going up in the last few years.
I am not diamond dealer, but I have many associates in these industry. I have been to over 100 diamond trade fairs overseas and have visited over 10 Debeer sightholders, and 40 diamond polishing factories.