The reason air bottles are more expensive is due to the pressure they need to withstand.
Under pressure CO2 forms a liquid which creates an equilibrium inside the cylinder. So as you squeeze more CO2 in there, more of the CO2 turns to liquid, keeping the pressure roughly the same, usually 800 - 1200psi.
Air on the other hand does not condense under these conditions. So the more air we squeeze in there, the more pressure the cylinder must take, typicly up to 4500PSI.
Also, every air cylinder needs a regulator on it, to reduce the pressures from 4500PSI tank pressure to a usable output pressure, often 850PSI.
CO2 bottles only need a simple on-off valve, because the tank pressure is already low enough to feed a paintball gun.
Getting hold of pressurised CO2 can often be cheaper than air, because CO2 is a waste product from several industrial chemical processes, so we (among others) are an effective means of CO2 disposal.
Although air exists all around us, safely creating the pressures we use, requires alot of energy and technical wizardry that is better left to the experts such as HPAC, which is why its more expensive.
