The proto-type was supposedely 300hp the actual dealer ordered GTX was not. No way can a CAI, cat-back exhaust and 3.5" pulley gain you the needed 60hp to reach 300.

10k is too much imho.

Consider that it's a seven year old car by now. My 97 only netted me about 3k. I would offer 8-8.5.

found website and sorry it was 260 hp and 295 ft-lbs. Look to the bottom of the page.

http://www.conceptcarz.com/vehicle/z751/Pontiac_Grand%20Prix%20GTX%20Ram%20Air/default.aspx

Well, it's already been pointed out that the SC'd GTX was only claimed to be 260HP, so that's covered.

I'd like to bring up a different perspective.... At this point in time, the GTX isn't worth a whole lot more than a GTP unless it's a GTP enthusiast(few and far between) If you plan on holding onto the car for 20-30 years and not driving it, you might have something to brag about.
With that being said, check Edmunds, and KBB for the price of a comparable GTP and go from there. If you can get away with what seems fair to you to obtain the GTX then go for it.

As with any car..... It's only worth what someone is willing to pay for it.