@zatnosk I don't know what a gemini terminal is, but aside from the screen not much, if there was manufacturing ability. So like a maker space with a laser cutter and/or CNC machine to make the case.
Ignoring the case and the screen, expect around 20 euros, a bit more if you want fancy buttons, if you can get to a maker space the case would be something like another 10.
Then screens are between 10 and 60 euros, they are priced weirdly so both eink and lcd monitors are in the whole range.