http://www.theguardian.com/world/2014/oct/01/drones-police-force-crime-uavs-north-dakota

"Frazier is a proselytizer for law enforcement drones. He believes they add to the police officer’s toolkit in fighting crime. He also stressed how cheap they are to run – when he worked with manned police helicopters in his previous job in Glendale, southern California, the unit cost $12m a year to sustain, while his current drone unit in Grand Forks costs just $10,000 annually.
As interest in drones has grown among police agencies, so too have fears about the downsides. Debates have raged across the country about whether the technology could lead to a vast expansion of police surveillance on individuals.
Such concerns have been fueled by plans floated by police departments for blanket surveillance. The city of Ogden, Utah, came in for a lot of criticism when it proposed putting a blimp semi-permanently on patrol."

This is a good article covering several angles of quadcopter use.So long as the police have rules to go by to protect privacy, quadcopters are a great cheap tool for the police and many other professions to use.