challenge for an engineer/robotics person: build me a butterfly decoy!

I'm a student in biology and I need a moving butterfly decoy. Your mission, should you choose to accept it: build me a decoy with moving wings. It must be battery or solar powered (I'll be in the field so I can't plug it in), and to make it more cost-effective than commercial models, it must cost no more than $20 to construct. I need a lot of these (5? 10?) so the cheaper the better.

The design could be similar to this toy butterfly, which uses a thermistor: http://www.entomon.net/3-monarch-butterfly-moving-wings.shtml OR http://www.ffandf.com/cgi-bin/cart.cgi/eb01.html (Conveniently, the first website even provides a diagram of the moving parts!)

Or you could use a motor or some other ingenious method to get the wings to move at a slow-ish rate similar to the one shown. It's up to you!

If you're interested in helping me, or if you have any good ideas for how to do this, please send me an email! I'd appreciate any and all the help I can get! If you can actually build this thing for me within the next week or two, we can discuss compensation in money, food, gratitude, etc.