Hi Scott:
If you really want to determine the value of these trees (i.e., firewood, low, average, high), then please contact a professional consulting forester for an on-site appraisal of these trees.
Nobody can provide you a value estimate for potential high-value hardwood species like black walnut without an on-site inspection of the individual trees. Why? Because stem/log quality, or field-grade, of hardwood species must be determined in order to adequately assess overall value for each individual tree.
Directory of Illinois Professional Consulting Foresters:
http://web.extension.illinois.edu/forestry/pdf/tfb-nres-202.pdf