There is not standard answer to this question. It really all depends first on the types of systems your area uses. I see you are from plymoth, but which one? There are more than one in the u.s.
If your area is old school you could work with a simple ans cheap analog conventional reciever that starts around fifty to a hundred bucks. If they are using a analog trunked system you would have to get a scanner for that, and if they are full blown digital youre going to have to spend three hundred at the very least. Some agencies and areas are complety encrypted or unmonitorable so in those cases not even the best most expensive scanner will do you any use.
If money is not an option i would say go for a digital trunktracker, such as the pro106/197, homepatrol, or psr500/600. They start at three hundred and go up to over five hundred.
It would help if you can specify exactly where you live and what agencies exactly that you are trying to monitor so we can reference the database and make the right recomondation for your area.
Also, even if your agecies use an older system tiday, more and more are transitioning to expensive digital systems each year... so what you may be able to hear on a cheap analog scanner today may go silent as systems upgrade. That is why i recomend a digital scanner from the start so you do not waste money on something that may not function a year or two down theline.