A 40 Cubic foot bottle of Argon, CO2, Argon/CO2 75/25 or helium usually lasts me at least a full roll of 10 lb wire on a Mig welder, but I can go through about 30 lbs of aluminum or stainless filler rod with the TIG.
Each bottle costs me about $30-$40 to fill.
Aluminum and stainless filler rod run about $10-$15 per pound.
How much you will use, depends on how you weld.
I don't use alot of filler with aluminum or stainless TIG, but can go through quite a bit using the MIG welder.
Anything under 1/16" sheet gets a little tough to weld with a MIG welder, but can be done.
Tig is alot easier to work with when you get that small.
I like to use the MIG on exhaust and heavier stuff and the TIG mostly on aluminum and stainless.
To get a cost per foot is really difficult, because everyone welds different and consumables are different.
One bit of advice, I would get a whole bunch of scrap aluminum to procatice on before you start on your project metal, because either TIG or MIG, it is real easy to blow through the metal and until you get the hang of it, you don't want to be blowing through your work metal, cause it sucks to fill holes. : )