So, does it...
... make its own way to the local service station and fill itself up when it "gets hungry"?
If not, then having personnel refuel a fleet (flock? Gaggle/Skein/Murder?) of these things every hour of air time is going to get costly in time, effort and wages.
I bet they want it purely for the "intimidation" value - nothing says "we're serious about using technology to catch dangerous drivers" more than having several thousand dollars worth of airborne spy hovering above the road - too bad that the departmental budget would be pretty-much cleaned out.
Whatever they invest in these things, they would get a lot more coverage if that money were spent on far less showy and less complicated devices - like cameras on poles.
FFS, a traditional ground-based Dalek body equipped with a camera would suffice if they really wanted it to "be mobile and look high-tech".