In total it cost me £91, which included two packs of this stuff http://www.ebay.co.uk/itm/Silent-Coat-S ... 5d4e7bd477 and a small paint roller from Poundland. I used all 42 square feet of the stuff and lined my doors, bootlid, floors, bulkhead and absolutely every panel I could get to.
It was totally worth the few days worth of effort, stripping out the interior and carpets, hoovering, cutting and roller-ing (and the £91!). I'd been considering soundproofing it for months and had looked at all sorts of websites/guides/any info I could find and came to the conclusion that I'd be better off doing it than not, which was completely the right decision. This also lead me to choose the 2mm Silentcoat stuff that I used, as it had good reviews and was far cheaper than Dynamat etc. Originally I order only one pack but soon realised I'd need two, but they delivered within 24hrs both times. I have since discovered I could have saved a tenner and bought a bulk pack for £80 instead of two smaller packs for £90. Doh!
My floor and all the metal I covered with this soundproofing was (finally!) free of rust, but I did leave gaps around most of the soundproofing panels just so I could keep an eye any rust that wanted to pay a visit. You only need approx 50% coverage with this type of soundproofing apparently anyway, so If anything I have actually used too much of the stuff.
Before laying it down, I had to remove a rather half hearted attempt to soundproof it previously. This was a slab of bitumen under the front seats, but this was quickly dispatched with a chisel, hammer and some heat. Once this was done I hoovered and used panel wipe on whichever surface was getting covered as I went. When applying it, because I was doing it in a cold garage in January I did warm the soundproofing panels with a hairdryer just so they stuck down better, as the packaging says not to apply below 10 degrees C. Applying them was quick and easy, and once gone over with a roller, they were stuck down good and proper, even to panels that were upside down. They were easy to cut to shape with a stanley knife too but the aluminium foil on the top layer of the soundproofing is aggresively sharp and I did manage to cut myself a few times. So if you do ever soundproof your own car, be careful!!
It took two and a half days in total, including tea breaks, ordering a second pack of the stuff, naps etc etc. So if you had all the materials to hand, from stripping out the interior to putting it back in again soundproofed I recon it could probably be done in about 12 hours.
The difference it has made to my car is astonishing!! 30mph is virtually silent and you can go along the motorway at 60 all day long without feeling like your ears are going to bleed. The doors sound far more solid and shut with a definite thud rather than clang, and it just feels more solid as there are now no rattles, booms or anything of that sort. My car didn't have any soundproofing or horsehair matting under the carpets to begin with (apart from the aforementioned lumps of bitumen) so this is probably why it made such a difference. I'm probably going to get rid of my daily driver, and use this as my only car soon too, which I think was the final push that led me to doing this.
I'm not affiliated with them or anything like that, I just wanted to share with you what I've done. If you're considering soundproofing your car, it's absolutely worth it. Best of all, I can still hear the exhaust "parping"

Didn't take many pictures, but you'll get the idea[frame]