You can do it one of two ways. And both mainly come out to the same end result. It's just some people perfer to do it one way than the other. Hourly and by area.
We are not into spraying yet but nothing will change much when we do as far as calculating labor. I would suggest you charge for labor and materials seperatly. I would figure out what you want to make an hour, Which will include two things.
General labor rate in your area for the service you are providing.
How much you want and need to make for your business to be successful.
You may have to start out using both hourly and aera calculations to give somone a price since it is a little different that mowing since you are actually applying a product down. With mowing you basicly have to figure in equipment wear and tear which is a little different in this case.
So with this you need to know how much your cost is for spraying each product down.
Tips to factor in yur cost (May need to factor in others depending on where the business goes)
Hopefully thats helps a little.