Picked up this K'NEX "Tethered Power Pack Motor" for $4 at a local hobby shop. K'NEX's site lists it at $17.57 . Runs on two AA batteries in the included bgattery box that includes a 3-way switch - foreward, off, backward. Though tethered to the battery pack, the "drive" part has two "power take offs" - one through the body and one off the end opposite the power cord. This thing is geared pretty low and should have enough torque to move a lot more wood than the HEXBUG micro robots I've played with earlier.
Here's a photo of the piece on quarter inch grid paper.
Here's the idea for the first piece that'll use this mechanism.