+1 on leasing.
I'm in southern IA, and we can lease a CO2 tank for less than $15 per year, plus refills from Air Gas.
I bought my first tank, for a great price. I think I paid around $50 for a new aluminum 5# tank. Which I promptly had to exchange for a crappy steel one because I can't get one filled here.
You'll be money ahead until year 5 by leasing a tank, at which point you'd have to have your tank hydro tested for $35, which would make break even around year 7 for buy V.S. lease
Another perk to leasing is that you can get a 20# tank later, if you want to.