It depends on your headunit, if it is an aftermarket 'camera ready' stereo then you can get a number plate camera for under £20, some decent wire and a better shielded video cable for a fiver then you're set.
If you have an OEM HU then the camera cost is way more.
I have had both sensors and cameras and see the merits in both, if you can get the OPS display on the screen then it is even better than just a beep.