Most of the backup cameras that I've seen use obsolete standard definition cameras with poor dynamic range, meaning that you get a pretty poor picture much of the time. The ones that come with a half-dozen low power IR LEDs are so weak that you can't see beyond 3 feet with them. IR lighting is costly, and it takes a $700 IR light to really be useful.
If you have the money, get a respected security company to custom install a high quality, wide dynamic range camera that can see to near-darkness all by itself. You can always switch on your backup lights for more light. They can hide it in your trunk or tailgate lock hole, and places that you'd never think of, but can give you a much better perspective than down low where your plates are.
If you're handy with tools, you can make a DIY backup cam with a good webcam and an app for your phone to view it.
If you're a cheapskate, well...you get what you pay for.