The weight gain depends on what your goals are, your budget, how carefull you are to treat each area just to the point of where it needs to be, etc, etc.
A basic install that I would do would add approx 22-24 lbs.
A pretty much all out install would double that.
Gonzo, just do not care about weight or budget, maybe three times the basic weight but you would have to be doing more than is ever really needed.
Somewhere in the middle of the basic and all out level is generally the best results for the time and cost invested.