Open source: Hardware design files (kicad) are licensed CC-BY-SA 4.0 and source code is GPL. Everything is on GitHub:
Parts cost ~$30. I haven’t designed a case for it yet though it follows fairly standard dimensions - it’s a common numeric keypad form factor other than a few extra keys and the RPI Pico board along the left side.
This is my second PCB design. I ordered 20 copies from JLCPCB - shipped to the USA for about $80 after shipping and tariffs. Before the recent tariffs I was able to order a similar board for about $30, sadly that era of cheap PCB availability seems to be over.