While there are a lot of cameras for $50 or less, and a lot of people who will swear by them, you will find that most serious forum members here don't trust the brands for reliability or quality.
If you are dead set on a $50 per camera limit, I suggest searching aliexpress for "Dahua 4MP" or "Dahua 6MP". I have a few of the older 4MP Dahuas like this and I have not been disappointed by them, given their low prices. At least this way you are getting a cheap cam from a major brand and not some little Chinese startup that has bulk ordered generic parts to slap together something unstable.