You'll notice an immediate difference even if you only put ~20sq ft per door. The main thing is to get SOME deadener on all of the large surfaces in the door, and seal up any holes as much as you can with the deadener and if possible something like mdf or plexiglass over the largest holes. When i started, i bought ~45sq foot and did both doors of my car with that. It made a HUGE difference, even though it was only barely enough to cover the 2 metal surfaces (outer door skin and inner door metal) of the door. If you can get 1 good layer over the entirety of those metal surfaces, they will no longer resonate anywhere near as much as they do now. It was a remarkable change... I was actually quite happy not to spend any more on deadener once those doors were done...UNTIL now..
I managed to score a roll and a half of RAAMAT BXT, about 5-6 yards of ensolite and 5 cans of the V&S 1081 adhesive from a local guy via craigslist yesterday for 150$ - and that makes it worthwhile to do some more deadening //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif
This roll and a half looks like enough to do my entire trunk (rear deck, trunk lid and trunk floor) and the inside of my plastic door panels for both front doors, with ensolite in the rear deck and in the front doors...i'll probably have ensolite to spare, which i'll stuff anywhere that i have small rattles that won't go away...