How much would a 15 carat marquise brilliant cut diamond cost?

Today on Blue Nile, the largest diamond available of this cut weighs less than half of 15 carats: 6.10 carats. It is an I colour with a clarity rating of SI2, meaning it is not a top-grade diamond.

For it you could pay US $71,674.

To find this cut in this larger size, best practices dictate that you search auction house options, such as Sotheby's or Christie's. Given their world-wide trade in luxury goods, you may see a diamond of that cut/ weight that you can buy.