Questions regarding "fair prices" to pay for corn silage always arise this time of year when the chopping starts. The University of Nebraska has a good guidesheet full of rules of thumbs and charts at: http://www.ianr.unl.edu/pubs/beef/g99.htm

There is a nice excel spreadsheet for pricing corn silage, developed by crop economist Ray Massey available at: http://agebb.missouri.edu/dairy/feed/Silagevalue.xls