The above Devin camera just sold on eBay for $1500, even though it's mainly just the skeleton.
I bet with $1500 you could construct a working prototype of a 3-color camera, optimistically.
In the pictures above you can see that there are provisions for fine tuning the placement & parallelism of the reflectors. Trying to build the mirrors right into the body without some kind of adjustable frame would be futile, don't you reckon?
When was the last 1-shot camera made? Consider the advances in machining since then! I bet a CNC machine could eat a 3-color camera for breakfast. National Photocolor still exists and can do custom pellicles. A 5x7" one appears to be only $250.
Fun to think about...