You can never get RPi Zero at $5 in volume. It's always one per household, etc. It's a subsidized educational price. OTOH for this kind of board $4 seems like

You can never get RPi Zero at $5 in volume. It's always one per household, etc. It's a subsidized educational price. OTOH for this kind of board $4 seems like about the right price - RP2040 looks like ~$1 MCU to me.