In my area, Green Gas is about $18 after tax, and I can only fire off about 1500shots off a can. I can buy 15 12g CO2 cartridges for $12. Each one allows me to fire 200-250 shots in my CO2 NBB Crosman C11. If my math is correct, that means the CO2 costs $12 for 3,750 shots, and Green Gas costs $18 for 1500 shots. If I used only my KWA USP compact I can get almost 2,000 shots off a can, but CO2 is still way cheaper.
Also, as previously mentioned, CO2 hits harder, which can be a drawback if your field has a low FPS limit.
And, if you live in a cold climate, CO2 is less subject to cooldown effect, and it will retain pressure better at cooler/cold temperatures.