How much is a one carat marquise diamond color I to J Clarity I Gemstone C worth?

these days not that much.

Another Answer

Blue Nile is offering a loose stone labeled Marquise cut, 1.01 carat with a Very Good cut, J colour, SI1 clarity for $2,500+. Your indicator 'C' is unfamiliar. Also an 'I' clarity will reduce the value of your stone, since inclusions (flaws) are visible to the naked eye.

Finally, the diamond is only worth what someone will pay you for it.