We've seen many DIY plotter designs over the years, but when it comes to frugality, this one might just take the cake.
The BAWP (Big Ass Wall Plotter) uses some fairly unconventional materials to make a large-format wall plotter at an incredibly low BOM cost. The main driver for the higher price of larger CNCs is that longer extrusion, linear rails, and ball screws are needed to accommodate the larger effective area.
Cory Collins solves this problem by using off-the-shelf electrical conduit from the hardware store. Combined with a couple of electrical outlet boxes and their pre-spaced mounting holes, he gets two very parallel and very strong rails for just a few bucks. Make a second and now you've got two axes!
The carriages move along the conduit using a stack-up of skateboard bearings and a few nuts for spacing. Collins had issues with the conduit bowing in the middle as the carriage moved along, but this was easily solved with a few pieces of wood glued in place to keep the same spacing along the length of the axis.
The carriage for the pen is made from scrap wood out of an old birdhouse kit. Attached to this is the ubiquitous 28BYJ-48 stepper motor. With a little 3D printed rack and pinion, he's got a linear actuator that picks the pen off of the paper. For control, G-code is generated using Inkscape and sent to an Arduino Uno running GRBL with a CNC shield.
Collins seems to have a knack for finding really creative cost-down solutions, which he demonstrated quite well in Version 1 of the BAWP:
"If you look closely in the version 1 video, you can see I made the gears from a disk and a Walmart pill bottle cap wrapped in GT2 timing belt to act as gears. Hey, sometimes to gotta use what you got on hand."
You can read Collins' write-up about the BAWP here.