I have made many custom keyboards ground up from the PCB. I designed and manufactured two out of the three boards in this picture.
The Mechboard64 could be turned into an very inexpensive kit.
The PCB when ordered from JCLPCB (cheaper than PCBWay with faster shipping!) should be peanuts, like $7-9 each. Double it if you want them to solder on the 6 or so surface mount components.
The problem is the bent metal plate. This is extremely costly to have cut and bent in small batches.
Easiest thing to do would be order a flat plate from a service like SendCutSend and then use some kind of 3D printed mount on the inside of the case.
A flat metal plate with the cutout for the keys and stabilizers would probably run about $35-40.
All in all extremely doable.
I probably will do it, but I am working on two other custom keyboard related projects at the moment. This will come next.