Right now at the coop near me I can get my 20 lb tanks filled for $14.25. At the exchange they are $22.00. That's quite a savings. As far as how long a tank will last a single burner forge will use (approx) 1 lb per hour @ 3 psi., 2 lbs an hour @ 6 psi, and so on..... Another thing that will make a difference in fuel usage is the type of insulation in the forge. Any gas forge should have a Reflective coating in it,(plistex,Metrikote,ITC 100 HT) if not get some. it makes all the difference in the world.
I just finished building a (single burner) gas forge, and cannot believe how well it works. I get a hell of a lot more heat from that single burner, than I ever did with the 3 burner, which will now be getting a Metrikote treatment.