There were at least 4 or 5 booths at the farm show in Des Moines this week that had spraying drones.  One of them was a company relatively local to me and apparently they used it to spray last year.  Unfortunately, they only have one drone and said they can cover 100 acres in a good day.  Application cost was $11/ac which is in line with other aerial application costs I've heard locally (and cheaper than local helicopter application cost).

Mike