Costs and benefits vary widely depending on the crop, acreage covered, swathing accuracy achieved, and other factors. To make cost comparisons, consult Investing in GPS Guidance Systems? VCE publication 448-076. http://www.ext.vt.edu/pubs/agecon/448-076/448-076.html. As a rule-of-thumb, a navigation system could cost six times more than a foam-marker system, which means that justification for GPS navigation over foam markers must be computed from the benefit side.