I assume you mean valve stem seals! That would be around 1000.00 at the Ford dealer, give or take, depending where you go. Probably wise to shop around a bit.
I don't know how many miles you have on your car and what your problem is. Judging by your question I assume (again) that you have a white puff of smoke at start up or right after idling...this is indeed caused by hardened stem seals, very unlikely that you have a piston ring problem.