here's a photo of the completed circuit board
The present component list is:
1 2N2222 or equiv NPN transistor
2 SN754410 quad half H-bridge chips
1 LM317 voltage regulator
1 10uf capacitor
...so about $20 worth of stuff.
Here's a short video of the whole thing in action: http://player.vimeo.com/video/33758255
This hack is not too expensive (the most costly part is buying the actual turret) or overly complex, but does require a degree of soldering ability. Circut construction for this hack requires soldering several components and wires. If you are not comfortable with soldering, you may want to get some practice first, or have someone help you who can solder well.
I used a Radio Shack #276-150 perfboard for circuit construction. I could have made the layout much more compact than I did, but it would have made the soldering even more difficult for others who might try to build this. The Radio Shack board already has lands and traces etched on it (indicated by the white silk-screening in the photo), making layout and assembly somewhat easier.