If you plan on going the route of home improvement store insulation material, keep in mind that almost all of it is asphalt-based and therefor will continue offgassing harmful and often smelly fumes for the life of the product. I know for some people that's not an issue, but living out here in the desert and opening my door every day to the smell of freshly paved road isn't that appealing. The products that are actually made for this purpose are typically butyl-based.

Dynomat was one of the first big names using this older technology in an automotive application. Since then, there have been a number of companies that produce a very similar and competitive product for almost half the cost of dynomat. eDead is an example of a quality, produced-for-automotive-application sound deadener that can be had for around 130 bucks for 100 square feet, free shipping on most stores on the web.

And also, as another member mentioned, sound deadener does NOT need to be applied to every surface of the vehicles interior. There is a crossing point of diminishing return by doing this. It increases weight, is costly, and after a certain point, doesn't reduce panel resonation or noise dampening. Something to consider.