Big companies mostly have a flat "stop charge"; it is supposed to cover your overhead and the cost to arrive at the location. Then they add a charge for each 1000 sqft (this covers your average product cost and your labor cost to apply it.) If you are licensed--this is skilled labor. And some companies discount big lawns--not much reason to discount an acerage, as you must apply the same amount of fert, and it takes the same amount of labor per 1000 sqft. Spread an acre with a hand spreader when it is about 90 degrees--you will understand it better. If you use a machine you must charge the customer for the machine time, (repairs and gas, plus payments or depreciation).