123: basically what it involves is setting a magnet up on teh forks of the front wheel, and another on one of the spokes on hte bike. a wire runs from the forks upto the mount for the computer, usually on the handlebars. the comp is programmed, by rolling it 10 yrds or somesuch, and then it can keep speed and milage pretty good.
prices vary, on speedos... i'd check performance bikes for some prices.
edit: I looked here:
http://www.performancebike.com/shop/...ID=4110&Sort=1
prices ran from 9 bucks to over 600. eek!