Fair, but do realize that sometimes you are paying for the "name" dynamat is the 3M of the sound deadening world. Kilmat, FatMat any other foil backed butyl product is more or less identical ( assuming mm thickness ). There are a few very small differences but you are really splitting hairs.
As for products that are made of closed cell foam, those are different all together. They are almost always cheaper but do not have the sound deadening properties that the butyl has. Sound is vibration, stop the vibration - stop the sound . That why butyls is so effective, its very dense, and heavy while still being fairly thin, and does a great job at stopping vibration. The cell foam products are literally made of a bunch of small air pockets and its very light and a lot thicker . It may change and dampen some of the sound but no where near as well.