No doubt so, but how about the cost of operations in rough country with poor access, where going in on foot is feasible (witness the illegals crossing it) but patrolling in ground vehicles is not?
Hence I think the real comparison should be: How does the cost of using a drone compare to the cost of using a helicopter in those same areas? I'd guess the drone is significantly cheaper.
Second, how long does it take a drone to patrol, compared to a manned ground vehicle in the same area? What's the total patrol cost per hour for drone vs 4x4?? (Don't forget to factor in the cost of the 4x4 as well as for the drone.) In rough country, a drone (or helicopter) can get an overview in a few minutes, but a ground vehicle might be forced to wind back and forth for an hour to reach the same point (and might still not get a view of the ravines). If patrolling a given area takes the drone ten minutes and the 4x4 an hour, which one is more cost effective?
How does it affect man-hours? The patrol is generally two men, while the drone only needs its operator.
How does all this affect insurance rates on their various equipment? Do reduced hours in use also reduce rates on 4x4s and such? (Certainly it will reduce maintenance costs.)
Lots of factors to consider, not just 'dollars per arrest'. We need to see spreadsheets and balance columns, not assumptions.