I measured a 30rnd SVI mag as being able to hold about 7g or propane which is able to propel about 55rnds if you don't fire too rapidly. Assuming no propane loss during filling, a 465g tank should be able to fire 3600rnds. I'm not sure if the Tippmann numbers are right, but 50,000rnds seems very hopeful. On the other hand, a Tippmann PEP has a significant expansion chamber and no slide to cycle.
Conversely, a GBB has a very small gas space which means that the propulsion cycle is probably a lot less efficient in comparison to the Tippmann gun. You get a pretty significant pressure drop when rapid evaporation is involved.
Want nearly free GBB gas?