i love googling a question and seeing the answer on future4200 
I can get 420ml cans x 72 for 230$ shipped from ebay
my math says this is enough gas for 10 pounds of material (7 cans per #)
https://www.ebay.com/itm/72-Cans-Whip-It-420ML-WHIP-IT-REFINED-Premium-lighter-fluid-butane-Torch/182738889006?hash=item2a8c16292e:g:P7MAAOSwNqtZpdkU
I just drove 2.5 hours total to get a 20# 70/30 tank with a 50$ tank deposit that I will most likely never return for 265$ (315 total) lets just assume theres no deposit for simplicity sake
This 20# tank can only do 5 pounds total? Thats over double the cost for it in the tank.
Id much rather tap the cans