I'm life long friends with with family that owns a 80,000 acre grain and soybean farm, and they looked into drones last year or so for crop assessment, but found it wasn't really effective from either an analysis or price perspective. The drones didn't really have the range or endurance to properly sample the fields (we're talking fields of several square miles each), and the resolution of the camera just couldn't give you the detail you needed. It was basically just a waste.
Perhaps if there were more specific imaging sensors on the drones or something, you'd have something, but a simple visible light camera just doesn't reveal enough info.