CO2 Cartridges can get expensive. I usually pay between $5-$10 depending where I get them and how desperate I am.   The bike shops are going to hate me for telling you this, but you can go to Wal-mart (or something like it) to the paintball or airgun section and buy a pack of 10 Co2 cartridges for less than $10.  They won’t be threaded so you’ll still need to visit your bike shop to a purchase a valve adapter to fit the non-threaded cartridges.  I’ve seen many of these to choose from.

Source: http://cyclingtipsblog.com/2009/01/co2-cartridge-alternative/