My brother managed a custom harvesting crew for a few years. I believe the rates were from $30-$45 acre depending on what part of the country, the crop being harvested and what elevators they were hauling too. Their rates may have been on the higher side since they ran new equipment, but they got sh!t done. In July 2013 they harvested 16,000 acres of wheat in 13 days. They pushed hard from the beginning of May into November.
Sometimes you can find and old local farmer looking for some work that would probably be closer to $20 an acre.
Spraying can vary from around $8-$12 and acre based on some numbers I heard a year or two ago.