Calculate the pounds of nutrients and dollar value of the manure being spread. For accuracy, you need a manure analysis, and the current fertilizer price/ton. As of April 2018, prices are around: nitrogen ($0.49/lb), phosphate ($0.44/lb), and potash ($0.36/lb).
A very simple spreadsheet (manure value per ton or 1000 gal) is at Manure Value Calculator-13cmc4k
Two more thorough spreadsheets are from Cornell’s Nutrient Management Spear Program: “Crop Available Nutrients from Manure” and “Value of Manure Calculator” at http://nmsp.cals.cornell.edu/software/calculators.html
