If you're looking to tackle a home made Wireless Follow Focus project, you might want to give VImeo member Phillip James a holla. For the past few months, he's been working with Arduino to design his own WFF, and seems to have worked out all the bugs. It definitely looks like all the parts are in the right places, and appears to be very precise in movement. Take note that one thing inexpensive WFF's are known for is the amount of noise it can generate. WFF's have a place and sound should always be captured seperately (like on real sets).
If you're not up to the task of a DIY the most inexpensive one to first hit the market was from Jag35. Although it worked pretty well, it's been polished up quite a bit lately. Pricing is unbelievably cheap if you've ever shopped around a for one. Prices are available following the link below.