One can purchase ready-to-go ultrasonic sensors for $30 such as those from Parallax or Devantech but for max-cheapiness you may want to roll your own as this fellow from Micro Examples has done. You can make a basic one by having your already-on-board microcontroller do the timing.
Theres a lot more nice PIC projects with schems and code there, so hang out for a bit!
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