Sound deadener isnt really something you can do cheaply and have good results.
At a minimum its going to cost you a $1/sq ft, but that price can as much as triple when you buy quality stuff.
Its a lot of reading, but this will basically explain the pros and cons of different deadeners.
http://www.sounddeadenershowdown.com/
Its a labor intensive process, but it is helpful, and realisticly its not that expensive to do it right. 100sqft will be more than enough to do your doors and probably most of your trunk, which is where most people start with.
As far as the under hood lining, it doesnt do much for noise, only thing ive noticed it helps prevent is the windshield from fogging up when its raining/humid.