How much would a letter of compliance on a vehicle cost?

I'm seventeen years old and looking at buying a car next week, more than likely a 2002 chevy cavalier. I'm moving to the u.s. in August and plan to take my car with me, what is it going to cost to get a letter of compliance from the dealership so that when I intend to transfer the insurance and registration to the u.s. I don't run into any problems?