I charge per field for labor and equipment, then add material, subtotal, and add profit and overhead figure to that.
There is an example in the football field renovation thread that was just moved to this forum. That field is one of the few I have done that was not mostly a sand mix. I used straight compost on this one. 24 cubic yards goal post to goal post. See photos for coverage. I use 24-28 yards on full size soccer fields.
I will see if I can back topdressing cost out of the invoice on the football field above and post it. Not sure how I feel about it, exactly.