How can a get a good but cheap gaming computer

In my experience the most cost efficient way to get a good gaming rig is to build it yourself. Building a computer is relatively easy, and parts are widely available at online retailers like Amazon, of eBay.
If building a computer is too complicated, or time consuming consider buying used. You will pay a large premium when buying a new computer, simply for being the first person to own it. Find the minimum system requirements for the games you would like to play, then search for a used machine that meets, or exceeds those specifications, look for used computers on Craigslist, eBay, Amazon, or the classified section of your local paper. Computers become outdated rather quickly, so buy the best computer that you can afford.