HPA High presure tanks, a 3,000 PSI tank would normally run you $75.00. To fill it depends, some charge $4.00 some more.

HPA is short for High Pressure Air. It is simply the air around us which we breathe (or in some cases pure Nitrogen) which has been compressed to a very high pressure, commonly 4500 or 3000 psi (pounds per square inch, measure of pressure in the tank). When stored under pressure, HPA retains its gaseous state, unlike CO2 which liquefies under pressure. This means that with HPA, there is no need for any liquid to "boil" for propellant. Since CO2 is reliant on external temperature to boil (it changes state endothermically) CO2 is problematic in cold weather or in extended rapid fire which will cool the tank and possibly introduce liquid into your gun.

For a further, more detailed explanation, I recommend you read Jaron's thread on this very subject.

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