Under $200. Single 2S LiPo battery. Readily available parts. Programmable flight control via Python.
BBBmini is a state of the art Linux autopilot / flightcontroller Cape for the BeagleBone. It can be used for Copter, Planes or Rovers.
Take the eduMIP and hook it up with the DSTR app in order to control the Mobile Inverted Pendulum from your very own cell phone!
Graphical Systems Design approach to controlling the Renaissance Robotics eduMIP with LabVIEW