Depends on where you draw the line. I think one can be made for under $50 USD. MCU: $5 PCB: $5 Sensors: $8 Motor driver: $3 Battery: $5 Misc. electrical comp

Depends on where you draw the line. I think one can be made for under $50 USD. MCU: $5 PCB: $5 Sensors: $8 Motor driver: $3 Battery: $5 Misc. electrical components: $5 Motors: $4 3D printed parts: $15