Sorry, but the price I "specced out" was with the modules from Adafruit. If you actually made a custom PCB, etc, you could easily hit the $50 mark with only 10s

Sorry, but the price I "specced out" was with the modules from Adafruit. If you actually made a custom PCB, etc, you could easily hit the $50 mark with only 10s of units, in my opinion. Especially since the processor is unimportant: there are no complex calculations to perform.