Answers in order of questions:
Cost: the gearmotors were from Surpluscenter.com when they had them in stock. They were $15 bucks a pop I believe. Unfortuneately, they don't have them in stock any longer and I only got the 2 back then. I did a quick internet search for 50-60 rpm DC gearmotors and saw a source that looked like them, but they wanted $35 each. I will search around some more.
I used a speed control which was around $40, but a person could use a couple servos to activate a set of switches instead which would makes things less expensive. I don't think being able to run in reverse will be feasible in grass anyway, but it is fun on a floor.
You would most likely run a 4 channel RC radio on 75 or the 27 Mhz range. Not sure if there would be a cheap RC toy out there to run it instead.
Other parts are some aluminum bar stock, a couple of swivel joints and some scrap wood laying around.
I would say mine has somewhere around $210 worth of parts including the RC setup with speed control and custom Nicad battery as shown in the video. I robbed the electronics from a different hobby that I did a couple years ago.
For a How-to, not sure when I would get around to one. Dave's photobucket page shows some good close-ups on his. Mine, just uses 2 motors (and about 10 times the rpm) for control instead of the 1 motor that he used shich would only gave his a straight line run movement.
Here is that discussion from halloween forum on Dave in the Grave's version that got me around to finally building my own version:
http://www.halloweenforum.com/showthread.php?t=51056
