Cheaper? Not sure. I recall a thread here once about using drones for agriculture where someone pointed out that they can rent out an aerial op over their fields (not sure whether it was spraying crops or just photographing the field with IR cameras and stuff) for less than it would cost to get a single drone in the discussed project, that would likely require lots of those drones to make sense. Agricultural plane rental in the US seems to be cheap, like taking a cab every now and then is much cheaper than buying a scooter for every member of your family.

Good point about the safety. That said, a nice Ag Cat or Snow AT can carry a hefty load of those pellets in the hopper and spread it with venturi or even a centrifugal broadcaster. Probably a ton or two or more?

Yeah, that was my thought. Cost per hour, drone wins. Cost per pound, above some limit, the plane wins.