Money is tight? Don't forget to factor in the cost of replacement cartridges.
What size tire are you filling up? That matters in what size cartridges to buy.
Make sure you get one that can fill up your tire to enough PSI. The 16g cartridges can fill a road tire pretty well, but they are expensive to replace ($4 each).
If you want to buy in bulk, this place has several options.
http://www.gas-depot.com/products.ht...=0&sort=normal
You can buy a box of the 12g cartridges for $5-10 bucks at Walmart. They will fill a 26" MTB tire to an okay pressure, but they don't fill a road tire enough. You'll have to break out the hand pump to top it off.