they are mad expensive, like 300-1000+ for the kind the police use, mostly because they have multiple transceivers to mount in the front and rear fo the patrol car, and advanced electronics to detect jamming errors, differences between the strongest signal and the fastest signal, or anything that may indicate an innaccurate reading of a target, this is important in upholding a speeding accusation in court.
You can however buy a radar gun that will measure the speed of ANYTHING for pretty cheap, i bought this one at radio shack a few years back, and the fun has never ended with it, i keep it in my car at all times. If i get pulled over for speeding i will question him and present my own radar reading, it never fails to make them laugh and let me go, also, you can use it to set off people radar detectors and watch them freak out.
This is the one i bought...
http://www.gosale.com/4272084/bushnell-velocity-radar-gun?gclid=CL3pqfKu2JgCFQrFGgodWUG_cQ
its made for sports like baseball, but it works on anything that moves, like cars, your dog, whatever