I have made a few custom keyboards with the stm32f103 ("bluepill"). You can get a 5 pack for $10. You get more IO pins and you don't need to suffer through an

I have made a few custom keyboards with the stm32f103 ("bluepill"). You can get a 5 pack for $10. You get more IO pins and you don't need to suffer through anything Arduino-related. The starter kits often include an ST-LINK clone that makes flashing much easier than dealing with bootloaders and USB serial connections. (I use a j-link, but both are fine.) It is, of course, supported by QMK. If you want to go even cheaper for a 4x4 macro pad, you can just hand-wire the thing and not buy a circuit board. I hand-wired a full keyboard and while it was not the quickest task in the world, it also wasn't that bad. Kind of relaxing. You also don't need to go overboard with mechanical supports for the keyswitches; I just 3D-printed a platform that the keyswitches slot into. Basically, if you happen to have some magnet wire, a microcontroller, and some Cherry clones sitting around... you can make yourself a keyboard. (Diodes are nice, but you probably don't need n-key rollover on a macro pad.)