Diamond value depends on a number of factors, such as the 4C's, Carat Weight, Cut, Colour and Clarity. Other conditions also influence price at a macro level such as production limitations and capacity as well as market supply and demand.
At this writing, Idexonline.com's average asking price per carat of a Round, 4/4 (1.00-1.49), D-K, IF-I1 diamond is $5800.49. Wherein Round is the shape, 4/4 (1.00-1.49) is the carat/weight, D-K refers to color and IF-I1 is for clarity.
You can also visit the following sites below for additional information on diamond pricing:
PriceScope
Diamonds Search Engine
The Diamond Registry
While each individual diamond price varies, here are some general price ranges using average color and clarity shared by D.NEA.
There are 2 basic categories in terms of pricing, those priced off of the Rapaport diamond price list and those that aren’t. Learn how to read Rapaport price list at Info Diamond.