I'm amazed that a standard keyboard build still targets the Pro Micro. Here is a build tutorial from 2012. 1Up (used in the article) is certainly more cost effective ($9) than SparkFun ($18).
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.)
[0] https://docs.splitkb.com/hc/en-us/articles/360011510839-Why-...