you have to be careful buying one used. The owner has to unmarry it from their ecu. If they don't do it correctly, then your screwed. I believe its three or four hundred dollars to send it back to Cobb, and have them re-do it.
I bought mine used, but at the same time, you kinda have to trust that the seller knows what the hell he's doing.
You probably wont see a legit used AP for under 500 bucks.