If you're doing the "roll-it-yourself" stuff (Herculiner or similar), just look at the weight on the can. If you use the whole can, you've added that much weight.
Minus the evaporating portion while it cures. I've done a roll on bedliner to my Ranger and I would guess the weight of the can is less than 25 lbs (I think I did the whole 6' bed with one).
Yeah, I doubt that the "added" weight would be noticeable. Probably bring that much extra weight back in the form of dirt and/or mud!