How much would guards cost responding to an alarm?

I’ve read that a couple of cities are refusing to send police response to burglar alarms unless the person/business being burglarized can confirm by a non-alarm means that a building is being burgled.
One option is remote video surveillance, but another is dispatching private guards to drive to the site and check for signs of forced entry from a “safe” distance.
Any of you dopers know what it would cost to have a detective/security agency perform manual checks?
Let’s say I was running a bank with 50 branches and wanted to contract for this service for a year.
What would it cost? Would I get billed per alarm, annually, or?
How about if I had a national bank with 500 branches?