I looked around to try to find the cheapest option for an Arduino microcontroller. I found one called RBBB (Really Bare Bones Board). I figured out that if I bought the PCBs and components separately, I would end up spending $7.24 per lock box.
I omitted some unnecessary parts of RBBB to save cost, flashed the bootloader myself, and wired it directly to the back of the LCD module like so. Also, it's hard to see in the photo, but I had to add small piece of protoboard to help with some of the wiring, and also to add a trimmer to adjust the LCD backlight.