green gas is a low type of pressure gas wich equals less fps.
propane is the cheapest and most reliable gas one tank is about 4$ and that can run ur seccondary for a year.
c02 is used for higher fps guns and is the most expensive 0.12g canisters
the only gun that wastes c02 is the TF-11 shooting over 1000rnds a minute.
the tauruse like i said can shoot aprox 200+ rounds on one c02 and is fairly consistant.
if you wana spend money on one seccondary get a kwj works gun they run on propane and are very reliable having metal slides and GBB