I'd say there are some very good suggestions, both with Raammat and Don from
this site have a ton of great information out there. I used Raammat and closed-cell foam I got from Rick, who was incredibly helpful, and MLV I sourced locally (it's really expensive to ship).
I spend about $300.00 on materials, going off of memory. I also spend about 2 week of evenings and 2 full weekends to do the job. I hate to think how much this would cost, but it would have to be a couple grand to pay someone to do, since I removed the entire interior of the car, all the door panels/roof panels, etc. There's no quick way to do this--it takes a lot of time.
As soon as anyone sets foot in the car who hasn't previously ridden in it, they comment immediately on it, almost like something's wrong. It's limo quiet, almost startlingly so. The nice thing is that I don't need crazy power to drive the sound system, and I typically don't even need to turn it up very loudly. I cant talk to people in the 3rd row if I ever have passengers back there. Overall, it's been a great investment of my time, since I go on a lot of road trips and I like to listen to music...and as they say, a speaker only sounds as good as the room in which you're listening. As cars go, I have a pretty awesome room.
It's a huge time commitment to do it all the way, but for me it's worth it. At a minimum, I'd do the front doors, front floor and the fenders and hood.
Total of materials added about 55 lbs. I'm pretty skinny, and so is my wife, so I figure we're still coming out ahead!