Specifically, this has a STM32F303xC. Looks like (depending on quantity, of course), you're looking at $3 to $5. Their other well-known keyboard, the Ergodox EZ, uses an ATMEGA32U4-AU. It's also $3 or $4 on Mouser. I don't know what the actual BOM looks like in total, but it seems like this isn't significantly different in price? Especially at the likely volumes here?

I'm still new to this game, so maybe I'm missing something.