You can go many ways; the cheapest would be using beer gas. I used to have a setup with beer gas, but the tanks are very expensive to fill around me and I would only get about 3 kegs for each tank.
About 2 years ago, I traded my beer gas tank for a pure nitrogen tank, and purchased a gas blender. Micromatic mm100 and Mcdantim trumix 100 are the most common gas blenders in use. Now I have a CO2 tank and a nitrogen tank. The cost to fill both tanks is about ¼ the price of a tank of beer gas, and lasts about 3-4 times as long. It will take a while to offset the cost of the gas blender, but the gas is always perfect and cheaper per keg.
This article also makes me leery of pre blended beer gas:
You can usually find a bargain on a used blenders on ebay, new they run about $700.