That's interesting, the first time i've ever heard of someone swapping a street driven GT to an auto. The cost depends a lot on where you find parts and if you can do the work yourself. You'll need the transmission, clutch, pressure plate, pedal assembly and computer as well as a few other odds and ends. You may be able to find the transmission and bellhousing in a junkyard, along with the pedal assembly. The clutch really should be bought new though.