Well, I'm pretty sure this is more than you want to spend, but it's a really good backup camera! I replaced my factory car stereo with an Eclipse AVN7000. It's not only a stereo, but also does GPS and has a 7" screen. There's an add-on backup camera available for about $150. The stereo itself is what really cost money. It retails at $1600 to $2000.

You can get just plain backup cameras much cheaper, but the problem becomes where you put the screen. Two popular places are built into your rear view mirror and built into your visor. Unfortuantely, our visors are too small for these screens. You can also mount the screen to the dash, but that's an ugly solution.

Another thing you want to consider is where you will put the camera. There's generally two types of cameras: stick-on and tap-and-drill. The stick-on cameras are obviously less invasive to your car, since they just use glue. They are easier to install. But they can come off due to weather, vibration, and vandalism. If you decided to use a stick-on camera, it's recommended that you install it inside your car, looking out the back window. That's where mine is. (right next to my center brake light)

For drill-and-tap variety you have to drill a hole in your car and bolt the camera in. Ususally they are put into the trunk lid, or next to the license plate.

I hope that helps. A visit to your local car audio installer will help too, since they can show you different choices and the quality of the screens and the view.