What is the price for half of carat diamond?

Diamonds are priced by the 4 Cs: Carat weight, Cut, Color and Clarity. You'll notice that weight is only one factor, making the question essentially meaningless. A diamond with obvious flaws and inclusions will cost much less than one with no visible defects, and a bluish-white stone is likely to be worth a lot more than a yellowish one.

For example, today on Blue Nile, you can buy a .52 carat round diamond, J colour, VS2 clarity with a fair quality cut and spend just over US$500. Or you can spend just over US2,800 for a .51 carat pear cut diamond, D colour that is Internally Flawless with a very good-quality cut.