I have no doubt that there are cost-efficient applications for drones. Maybe policing crowds at football matches etc - where it is over a relatively restricted area, for an extended time. I suspect they would not be effective at searching 20 miles of remote coastline for a misper, or searching a chunk of Snowdonia for a missing aircraft.
Unfortunately cost-effectiveness is not always easily quantifiable - if you launch 100 times and save one life - how do you measure that? worth it or not?
The protocol for vulnerable persons always used to be - send the helicopter, because it is better to send it and potentially waste a chunk of time & money as opposed to not send it and potentially waste a life.
If that protocol changes, it would represent a massive shift in ethos for the police.