As a Halloween prop builder, I'd consider building an animated scarecrow with a 12 volt DC windshield wiper motor. A simple crank arm to move the shoulders back and forth, or move the whole upper torsal in a circle. Add a motion sensor between the transformer and the power line to set it off. Kind of like the ground breaker I built last year that's 20 seconds into this video:
http://www.youtube.com/watch?v=moKWYXI_7Pk
Video showing early construction:
http://www.youtube.com/watch?v=ggk0X4wfkx8
It wouldn't cost much to build. $20 for a wiper motor, $10 for some PVC, Maybe $20 for a cheap power supply, a small plastic container for weather proofing the power supply, some scrap wood, clothing, wire, motion sensor, outdoor extension cord, and whatever else you might have lying around in your house.