I just designed and built an RPI-hat style PCB with JLCPCB that has multiple MCUs, power, and a 40-pin connector on-board. Each 100mm x 100mm board was $~10 a

I just designed and built an RPI-hat style PCB with JLCPCB that has multiple MCUs, power, and a 40-pin connector on-board. Each 100mm x 100mm board was $~10 a piece, assembled and delivered to the US (the cost mostly consisting of my specific parts.) It's pretty cheap, low effort to prototype a real PCB.