It's not that things get more expensive as they get smaller. As long as you're within reasonable tolerances it can be more cost effective. There is very little reason in a consumer-grade product, especially a devboard, to push any major fabs' tolerances. SMD components are grain-sized and you can make traces pretty thin.

For example, this project is a 2-layer PCB. Those are bog standard. With this small of a footprint it can be printed on a single surface and cut out. The schematic they posted keeps everything on a single surface for print. This is also an operation any fab can easily perform. If you order from China, even cheaper (even accounting for duty cost + S&H).

I'd be more concerned about the MCU and the components rather than the cost of a custom PCB.

Looks like it only has 6 discrete components [0], and the pinout on the MCU looks like a standard QFP [1] (and everything else is on larger pads), so I think that you should be able to manufacture this using standard production techniques/manufacturers. But I know hardly anything about hardware, so I may be completely wrong here.

[0]: https://github.com/user-attachments/assets/0f7962d5-38e1-4bd...

[1]: https://en.wikipedia.org/wiki/Quad_flat_package

This is backwards. PCBs get cheaper to produce the smaller they are, because PCB panels have a fixed size and you can put more PCBs on a single panel if they are smaller. Since you're repeating the same PCB over and over again, it means you're constantly placing the same parts over and over again, meaning that you need fewer reels, which means fewer machines and a shorter assembly line.

The really expensive stuff is through hole components and bulk components (relays, connectors, etc) because they need separate machines (wave soldering, large pick and place).

Placing small components is dirt cheap due to machines like SIPLACE SpeedStar. [0]

[0] https://youtu.be/3joTYHRTcCs