Here are cost figures on various farm operations based on cost accounting records of Iowa State University. The information can help you figure out custom rates suitable for you and your area.
Frisby, James C. and Hull, Dale O.
"Farm Custom Rates for 1964,"
Iowa Farm Science: Vol. 18
, Article 4.
Available at: https://lib.dr.iastate.edu/farmscience/vol18/iss7/4