It depends. I like to combine by the hour not the acre. Crops and conditions vary from year to year so much, that I found that penciling the combine by the acre was an excercise in futility.
We are charging our 9660sts out at 200 an hour and the customer pays for fuel on top of that.
When your crop is tucked away nicely in the bin what is it worth to you? The alternative could be a crop sitting out losing grade, or just plain losing it all. Then maybe quibiling over per acre combining rates seems pretty insignificant.
I calculate it on a per acre basis. In the 12-15 dollar range.
It all boils down to your cashflow situation on your farm, and the amount of debt you are carrying.
If you carry alot of debt then you may look into an older combine that you finance for less and spend more money on parts and repairs, or buy newer that has warrenty and then your parts and repairs are not an issue.
I calculate my cost of the combine
depr + repairs / acres harvested.
I know some guys who trade every year on a slightly used combine. The machine is field ready for them and their payments
are the trade difference. These guys calculate their cost by separator hour (150-175/hr)