Going rate here is $18 per acre for reasonably clean ground, 20 acres and larger, just weeds/tall grass. Heavy brush is a different matter.
SOME un-insured, non-legitimate business (ie, doesn't pay taxes, ect) part-timers charge as little as $30 an hour for smaller tractor/mower rigs. Full time legitimate operators charge upwards of $60 per hour for 6'/7' mowers behind capable tractor.
15' batwing behind capable tractor (here) runs from $90 to $125 average. Larger acreage (hence, less transporting equipment expenses) can run less.
I usually get $125 hr for 95hp tractor/15' batwing. Will discount SOME on jobs of more than 8 hrs.