I put my DIY pressurized system together for $130 total, not bad.
I have a 5lb bottle on a 55G tank. I have not had the tank hooked up long enough yet to see how often I will have to fill it. From others I have heard a fill up should last anywhere between 6-12 months depending on your aquarium size & bubble count.
I found in my area that welding shops only have used canisters (which you have to swap for another) and industrail grade C02. So I went to a store in my area that supplys resturants with C02, there I got a new canister that I can have refilled & food grade C02 which is cleaner (less oil) than industrail grade. Just something to think about.
Also instead of using a spice jar for a bubble counter I use a Hagan Ladder (diffuser/bubble counter) that you can get from Big Als for $10.