The police unit I flew with kept the helicopters up all the time. They were used to support the ground units, and covered a wide area. They could be anywhere in their area within about 5 minutes. It is unlikely they could respond that quickly from a ground alert, and the only additional cost is the direct operating cost of the aircraft (officers there, getting paid, etc.)
Yes, they did fly 7 to 8 hours per machine per day, and had to put a second machine up during the 'weekend' because of increased business.
I don't know the direct operating costs of the R-22, but it is probably much less than $300 per hour. These people are the only ones who fly it, they probably get very good insurance rates, etc.
I don't know about any UK operations and their budgets, but if you look at the costs of even one unscheduled turbine engine overhaul, it's going to put a pretty big hole in your budget.
We need to make sure that we don't let emotion get in the way of logic and facts.
I understand that the UK is finally going to let single engine turbine aircraft like the Caravan and PC-12 operate at night and IFR (only carrying cargo to begin with). Same thought process needs to apply to helicopters.