If you know any friends that work at Bestbuy, I'm told its cheap as hell there. From my own experience, car audio equipment is dirt cheap from Bestbuy (I got a ~50% discount on my whole system).

BTW, my trunk rattles like hell, but I found the only thing I wanted dynamatted was the cardboard fabric piece between the rear seats and the rear windshield. Be sure to dynamat that thing like hell, as my one layer is sometimes not sufficient. And stick it on there good, in cold weather it tends to lose effectiveness, I'm thinking because of less adhesive properties in the cold. Good luck.

BTW... what are you running?