I have not found an effective wireless setup that sells for under $700. The distance is too great from the camera to the cab of the truck for the less expensive ones. One fellow was able to get a wireless to work by running wires from the camera to the front of the trailer and mounting the transmitter at this location so the signal to receiver distance in the cab was reduced by 75%.
I have a tailgate camera on my pickup and much of the time it is worthless as any strong backlight, which includes the sky during the day, will darken the screen so much that I cannot use the camera. There are ones that have a sunshade and these are going to be more effective during daylight hours. These can be adjusted to show a hitch for backing up and tilted to show the road behind you.
Some cameras will work with certain GPS units so the same 4" x 5" screen can be used for navigation and for the backup camera. Backup cameras can be wired into the truck if it already has a cab monitor as well. For this type of work it is worth talking to a local installation shop (same guys that do car alarms and stereo installations) and see what they would charge.