The carat weight of a diamond decides its expense. The Rappaport value list sets the rules for Dallas diamonds and diamonds around the world. You may see a few diamonds that are valued above this list, and this is a direct result of the exceptional quality of the stones well beyond the carat weight.
The higher the price per carat (or PPC), the higher the expense of a diamond. For a 2-carat diamond you can expect to pay more than twice as much than for a 1-carat diamond. The reason being is because the price is not necessarily determined solely in relation to the carat weight. Diamonds that are on the higher end of the quality spectrum for color and clarity will be more costly, although stones that are cloudy or milky are not priced by this list basically because the value is not there.