How much is a 1 carat marquise diamond worth?

It depends on the quality of the stone. Cut, clarity, & color.

Another Answer

As above, much depends on the clarity, colour and quality of the cut. You can buy a one-carat, marquise-cut diamond today on Blue Nile and pay between US$2,800 and US$18,900.

If you're selling such a diamond, best practices dictate that you take the stone to a certified gemologist and pay for a certificate to determine where on the scale, above, your diamond might fit. Then, your diamond is only worth what someone will pay you for it.