The prototype is quite inexpensive:

It should be possible to build such a device spending less than $15 (if you have your own 3D printer, however an improvised case made out of cardboard or other materials could also work).

There is an added cost for each print, because of the need of printing a transparency film for the exposure mask, but that shouldn't cost a lot.

The UV paint/soldermask is obviously also needed, but it is inexpensive and lasts for long. A 10ml container costs around $2-$5. There are various suppliers, i am using "Mechanic UV Solder Mask" which is available in various colors. Such paint is often available in stores which supply equipment for electronic repair shops, as it is often used to repair soldermask layers (but it is not used too much for actual printing!)