Answer: I think the first step is contacting your local cooperative extension office (look in the phone book under county offices or do a web search for University of Minnesota cooperative extension). Extension agents there will have an idea of the potential productivity of the trees depending upon their age, which will help you determine their worth. You might also get a little background information from the following study done by the Virginia State Cooperative Extension: http://pubs.ext.vt.edu/446/446-011/PDF_Section2.pdf
Realtors are also good sources of information on the value of local orchards. Best wishes with your new endeavor!
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