If you expect to make a profit at custom field work, you have to consider all the variables. Many things work to make your job harder and must be considered when you set charges for your services.
Hull, Dale O.
"Factors Influencing Farm Field Costs,"
Iowa Farm Science: Vol. 21
, Article 4.
Available at: https://lib.dr.iastate.edu/farmscience/vol21/iss7/4