I went and looked it up as I really had no idea:
Amazon.com : 7 inch TFT LCD Digital Car Rear View Monitor with Waterproof Car Rear View Camera combo : Vehicle Backup Cameras : Electronics
It is a little under $40 for the camera and the screen - not the worlds best but works fine in my case and if you want you can always use a better camera. I had the same one on a RZR-4 and it held up well so that is really the reason I bought another one. Figured the picture quality is not as good as the one on our SUV but for $40 and it held up on a RZR, power washing and one time the kids "washing" the screen it was good enough.
The whole thing started out as an experiment - I wonder if these would work on my RZR because I can't see anything behind me once we add a cooler to the bed. Well it worked pretty well and I found I really liked having it. My wife loves it because she can see a stump or something low that is behind the machine that she could never see just looking behind her.