It depends on the shape and the cut quality also.
Try doing a search for it on a couple of the diamond vendors site. Or go to the pricescope main page and do a search from there.
Pricescope has it anywhere from $10-17,000 depending on cut quality, though that list has a couple of real dogs (58% depth and 60% table--that's the 10k one). For B&M retail, multiple that by 1.5-3.0.
The one D-IF of this size Jonathan kept posted has moved away (Nice ?) and HERE's it's brother. Amazingly, the sercah produced just one ideal cut D-IF (THIS ONE). I am sure there is enough demand for these stones. The numbers above sound right....
The prices on dIf range from approx.12,500-16,500 depending on the cut of the stone ,and if it has any flour.
check the web ,go on different sites and compare similar stones,cuts,and prices. that should give you an idea on prices.