Instructions
Look for a Core or Arcade Xbox 360 system. The Core and Arcade systems both use the same hardware as the more expensive Pro or Elite systems, but lack an external hard drive and headsets. The Core and Arcade systems can usually be found at much cheaper prices, and you can always add a hard drive or a headset later.
Go to your local used games store such as FuncoLand or GameStop. These retailers often stock used consoles. According to GameSpot's website, you can get a used or refurbished Xbox 360 Core or Arcade for around $160 USD.
Check online auction or sale sites such as eBay or Amazon. These sites often have listings offering used systems for half the price of new systems--some sellers even throw in extras like games and controllers.