With used electronics flooded on Craiglist or laying around the house, you can get a pretty simple backup camera for dirt cheap.
First you'll need a OTG converter cable... or just make one at home...
Install a program on your Android O/S phone or junked Craigslist one and you'll have a decent setup.
Even two cameras!
Infinitegra, Inc. | Android App "USB Camera"
* USB cables should not be longer then 10 feet or they'll need USB amplifiers.
Im looking to try this soon...
Webcam models need USB UVC capabilities
Linux UVC driver & tools