I built my own home security system with a Pi as the brains to control camera streaming and do face recognition using OpenCV. Is there a $5 microcontroller that can do all that? Admittedly, it's a big load on the Pi and I'm looking for more powerful single board computers, but that again eliminates $5 MCUs.

I have another Pi working as a drop-in replacement wifi router that routes data via a USB data dongle in case my main internet connection fails. This too may be too much to ask from a $5 MCU.