40 bucks for a plan to convert a milling machine to a CNC ?
Have a look at www.cnczone.com
There are a lot of ideas, tutorials and tips.
With a basic knowledge of mechanics and electronics (which you should have for such a project)
you'll easily can do it by yourself
However, I can't really remember but I think I saw those plans once on the net.
I'll have a little search for it.