I don't know about a cheap one but there are a few USB follow focus models for Canon DSLRs that do just that for a couple hundred $. And Garret Brown's original concepts for the Steadicam actually included a wearable eyepiece for monitoring! lol
Here is one story I just quickly found on it:
http://www.icgmagazine.com/wordpress/2008/07/18/stead-as-she-goes/
The model for the remote follow focus for a steadicam would be the intuitfocus:
http://vimeo.com/9827192
There had been attempts to DIY sth. like this. I followed this video:
http://www.youtube.com/watch?v=D3-P1rKCEEg
(but didn't finish it, I just can confirm the low prices for the parts)
Parts are cheap (~20 bucks all together). I like the whole 'rig' design in this video. Perfect for an EOS with Magic Lantern.
You must know: These cheap servo testers
> allow a rotation angle of 90°. So you can only follow part of your lenses' focal range. I.e. the Nokton 25mm rotates 270° to go from 0,17 m to ∞. But: Only about 60° cover 1m to ∞.
> will translate the position of the controller knob exactly to the motor, but there is no ease-in, ease-out, like with the zoom lever on a modern camcorder, so slow focus transitions are not possible.
There are digitally controlled servers available, but they are expensive. Ask at your modeling shop.