1) A portable electrophysiology toolkit. Strap it to a cockroach's leg, stimulate it, and it spits out neuromuscular action potentials. Try varying the conditions and see if you can get the frequency of the action potentials to change. This will run you around $110. (HT to Andrew Hires)
2) A microscope on your cell phone. It has a resolution of ~ 1 micrometer and is amazingly lensless. Apparently it will only cost $10 but probably won't hit the market for 5 years, which is lame. There are cool comparison photos with a conventional microscope (10x objective lens, 0.25 numerical aperture) but the paper is stuck behind a paywall and so I can't post them due to copyright issues. (HT on the concept to Jason Snyder).