How Much Weight Does Soundproofing Add ?

I don't know how many of you have ever thought about putting soundproofing into vehicle.

I have seen some online that looks like sort of a heavy gel. I guess what got me started on this was seeing inside the door of my old BMW.

Thing is, I wonder how much weight it would add. Do you have to do it all for it to work, or are there certain areas that give the most bang for the buck. (I think I have seen mentioned between engine compartment and inside of vehicle, whatever that wall is called, is best ROI.)

Does anyone have experience with this ?

Looking forward to seeing what group sourcing can come up with here !

Thanks !