A new CNC router will cost you about $7,000 visit:
Ummmm, not entirely true, for what you seek a CNC router capable of milling Balsa and other types of wood, you can build one for under $500, search the web, there are plenty of people who have done homebuilt CNC projects, I'll see if I can dig up some of my old links.
There are 3 basic components that make a CNC machine different from a standard power tool, and that is one a Stepper motor on each axis to move the toolhead to the appropriate X/Y/Z position, two a controller board to link it all up with a PC, and three software to control it all, all but the software can usually be come by fairly cheaply, and there are a few free CNC software packages out there, you just have to scour the web, there are plans and schematics on the web for homemade controller boards, or you can buy a premade unit from Geko or a few other manufacturers for a couple hundred bucks. If you only intend to use the machine for milling wood you can build the frame out of pine 2x4s, plywood, 1x4s, 4x4s, or just about any lumber you can come by.