[brittohalloran]

It costs $45 (+ Pi Zero W + Camera + SD card = approx. $90 total) and could be left somewhere analyzing video frames and communicating results.

[DerfNet]

There are quite a few budget smartphones out there for under $90, without even touching the used market. Could the software run on an Android device?

[sargun]

Like what?

[DerfNet]

https://www.amazon.com/s/?field-keywords=cheap+unlocked+smar...

[Tsagadai]

Redmi 4A. There are some very interesting low end phones coming out of China.