Rick (RAAMAudio)'s products are great and he used to cut NASIOC members a deal on their orders. It doesn't take much to make a big difference... I didn't spend much more than $100 in 2007 for a roll of BXT, some Ensolite, and adhesive which was plenty for the doors, floor, trunk, and under/behind the rear seat.
Like mentioned above it's less sound deadening and more reduction of resonance. The doors in our cars in particular are about a millimeter thick and weigh nothing, and have what looks like a post it note stuck to them for sound deadening from the factory.