How many shots can you get out of a GBB magazine using propane?

If you fill a magazine used in a typical gas blow-back airsoft pistol (Thinking about getting a WE full metal M9) how many shots can you get out of that one gas fill of the mag using propane. Can I reload the magazine a few times with bbs and not have to refill the propane? Thanks. Any suggestions as to what pistol I should get are also welcome but I would like to keep it under $100 and I am sort of partial to M9s but an open to other things as well.