compressed CO2

While I have not used it for airbrushing, I have used CO2 in the past. The expense is not in refilling the tank.  Most places it will be in the neighborhood of $10-15 depending on the size of tank you have.  The expense can come from the tank and regulator. You would have to check your local supplier for prices, but it should be approximately the same price as a mid-level compresor, only without the noise.

Last time I looked at this option I was quoted almost $250 for the tank and regulators, $15 per refill. I estimated that the tanks would need refilling 3-4 times per year. An outlay of $310 for the first year buys a really quiet compressor, the $45-60 dollar hit per year just increases the total cost of ownership. All prices are in Canadian dollars. Your results may vary.

Do you remember about how big the tank was?  I have old regulators and filters from a machine shop that my father-in-law works at.  So I'm looking at tank and adapters only.

No I don't. I remember the tank being a bit over 2' high, 20 lbs. is ringing a bell, but that might be how much weight my wife says I should lose. Then she looks meaningfully at my head...

Yeah, a 20lb tank is the one that is about 2 feet high.  If you have the space and the money, you may want to look atthe really large ones, like 5' high and 8" or so around, as I think they are only a little more to fill than the 20lb ones.

I'm a CO2 fan for all the advantages CO2 offers.

1.quiet.  2. no water spitting.  3.no power needed.  4. very portable 

Yes a new regulator is expensive however you don't need a new one. Get a used one. CO2 stays at about 800 psi in the tank so the regulator must be rated higher than that. I went to an auction and picked one up with a nitrogen tank for $25. It was from a welding rig and the regulator was raited over 3000psi due to the nitrogens higher pressure. I then traded in the nitrogen tank at the gas company for a 20lb CO2 tank. You want to use CO2 over nitrogen because it is cheaper. I exchange an empty tank for a full one when I refill instead of waiting on them to refill the same tank.