Dynamat is killer stuff; just ask any serious stereo freak. However, there are a couple less expensive options. One is Rammat, and another is B-quiet. Rammat is very good, the other stuff I'm not to sure about. If you want more info on either, check out the forums at
www.elitecaraudio.com as quite a few of the guys there have used them. Also they guy who sells Rammat hangs out there. I think his name is Rick. Good luck...