The OWI Robotic Edge Arm is a cheap and terrific 5-degree-of-freedom robotic arm that just costs $37
. It's a great kit to build with a kid and won't take more than a couple hours to finish. Out of the box, it can only be controlled manually with a remote control. You can buy a USB control interface
for it, but it's a waste of $27. It just lets you add a series of timed moves, but it's not really scriptable, you can't have it loop and because it just uses timing, there's no reliable repeatability.
What you want is real computer control, with closed-loop feedback for precision. Good news: it's super easy to connect an Arduino board to the OWI arm and let it do all the work! You won't even have to take the OWI arm apart or damage it.
Here's a video showing it in action, under fully autonomous Arduino control:
Here's what you'll need.
1) OWI Robot Edge Arm
2) An Arduino Uno board
3) An Adafruit Motor Shield
4) One potentiometer
and at least three little trimpots
5) Some perfboard
to mount the pots ($5)
6) Some jumper wires
($6) and pin headers
The total should be around $100. But you may have some of this already, and if not, you'll be able to reuse these boards and other parts for countless other projects.