You are compareing the cost of a used j to a new WRX. Thats just absurd.
What if I did it the other way around. A new J cost 15,000 dollars, and I could get a used WRX for 17 or 18 thosand since the announce of the STi. So really can you bridge the profomance gap with 2 or 3 grand? No
So the 7 grand number is a good number to try to use. But since your scenario was at 10 grand, take the cost of a used wrx, and to be fair ill use 18,000 and then add 2 grand worth of mods and .. you lose