A Google search reveals a price of $39 at various sources, and even $13.29 on eBay (though can't vouch for quality--it does, after all, need to be tough enough to survive a fall from the night-stand):
*
http://compare.ebay.com/like/281095574229?var=lv<yp=AllFixedPriceItemTypes&var=sbarBy the time you purchase the box, the motors, the wheels, the LCD alarm clock [that can be modified to provide an external signal that can be used to trigger the "running" function], then more parts for the next prototype, since you'll probably need to make more than one before you get the design right [as the first few prototypes will probably destroy themselves on the fall] -- you're probably into more than the tune of $39.
I would just buy one.
But, if you're determined, check out these for materials:
* Pololu:
http://www.pololu.com [check out their gear motors, and wheels]
* Sparkfun: www.sparkfun.com [Also, gear motors and wheeles]
* SeeedStudio: www.seeedstudio.com
* www.sdp-si.com
* All Electronics: www.allelectronics.com
