In the project description I wrote that I wanted the button to be "portable, small, cheap and able to communicate with openHAB over radio". It means that it must be battery-operated, low-power, rechargeable, with the least possible number of components and with a way of communicating via radio.

I will probably end up designing a PCB for it so if the PCB is as small as possible it wouldn't hurt.

My goal in terms of cost per unit is around 5$/€.