Add equipment to the mix.

For opportunity cost you should charge yourself custom rates for everything your equipment does.          Just looking at the ISU custom rates last night.

Spraying is $6+ per pass.      Planting is $16+       Combining is $32 (which is cheap compared to planting and spraying).        It doesn't cost us anywhere near that much to run any of those pieces of equipment.

For that matter figure the opportunity cost on your time.        If you're a 4 yr college educated business major could you be pulling in $50-100/hr working a much easier schedule???        If yes why are you driving tractors, mowing yard, hauling grain, etc????

You can go on forever with opportunity cost if you wish.