While Dynamat does perform according to it's intended use; you really are paying for the name.
The mark-up on Dynamat is discouragingly excessive.
Have a look into a few of these:
B-Quiet
RamMat
eDead
Cascade Sound Deadening
Generally you can get more than twice as much of these at a lower price than you would with Dynamat.
I'm rather an audio nut too, and have used darn near all of it. They work just as well, if not better than dynamat.
Or from Menards/Home Depot/Lowes in the roofing section:
Peel & Patch
Peel & Seal
Basically, it's the hidden secret that the sound deadening companies don't want you to know about.
This stuff comes in various sized rolls, is aluminum backed, and costs a fraction of the previous brands that I mentioned, and basically is 100% less than Dynamat.
Works quite well also.
I'd really stay away from the Dynamat, as these other products will do the job quite nicely, and definately at a lesser out of pocket cost.