Make a fully-functioning CNC machine that can accept different tools for less than $150 by using parts from DFRobot and Banggood.
I started by thinking of a free-wheeled machine that could have an infinite Y-traverse, but it proved to be too cumbersome and inaccurate.
The bed (Y axis) is comprised of two 500mm or 800mm V-slot rails arranged in parallel. There are four plastic pieces that hold two wheels each, allowing for easy travel of the bed along the rails. I also ran a timing belt across the Y axis under the bed, so that when the Y axis stepper rotates the belt moves.