There's not all that much to a RCM - a holding tank for the solution, an aquarium pump for the liquid, the cleaning brush, platter, motor, wand, vacume motor and another holding tank with a drain.
You could probably copy my VPI 17 for around $200 in parts.
If anybody does buy a VPI machine, do yourself a big favor and remove the liquid tanks and put some varnish or plastic or something in that area. They just bolted it to particle board and it will get wet and disintegrate and then you'll be cursing up a storm.