Propel Direct via Newegg offers the Hubsan Q4 H111 Nano 4-Channel Radio Control Quadcopter in five colors (Black pictured) for $17.99 with free shipping. (Battery Superstore via Amazon has it in Yellow for the same with free shipping for Prime members.) That's the lowest total price we could find by a buck. It features 360° eversion, a built-in gyroscope, 4-channel function, and 2.4GHz radio control.

Update: The Newegg item has sold out, however Battery Superstore via Amazon still offers it for $17.99 with free shipping for Prime members.