I have been doing tons of research, but am still confused about what parts go into building an axis, and how they fit together.

At the very least, I would like to know what would be the best setup for a small - Approx. 20x20x6 - mill that will be used mostly for milling PCB's, some wood, and possibly some urethane.
I would prefer to keep my costs to under $100-$150 per axis. That is neglecting the cost of motors, of course (which will probably be Pacific Scientific 144oz. in., NEMA 23, Dual Shaft)