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Total Length of project: Approximately 4 weeks of spare time. Total cost < $1500.
(Note that this video is of the generation 1 CNC machine. Like most people that build them, I immediately started modifying it. It's now on Gen 3. The X axis now has twin screws, and the Z axis has been completely rebuilt and is direct drive. It also has an air cooling system for longer bit life.)
A few good links to homebuilt CNC websites:
CNCZone
DIY-CNC yahoo group
Xylotex - supplier of good electronics for home/small-shop CNC