The dealer will probably not give you a credit at all. You can always go to a a place like America's Tire and to a trade for a credit. I traded in my stock Primacys ($240 total credit) with 1800 miles on them. Used that credit to buy a set of Michelin Pilot Super Sports. If you plan to drift on an empty parking lot every now and then, you will need to avoid the Pilot Super Sports because they are very "sticky". For more on tires check out this technical topic
http://www.ft86club.com/forums/forumdisplay.php?f=24