Greetings David:
The most efficient way to value a stand of timber, or even several black walnut trees, for potential sale and marketing is to contact a professional forester.
Therefore, your best bet is to contact a local professional consulting forester:
http://dnr.state.il.us/conservation/forestry/CONSULTING_FORESTERS.pdf
Alternatively, you may contact your local IL Dept. of Natural Resources district forester; however, they are usually backlogged with forestry requests upwards to six months--two years.
http://dnr.state.il.us/conservation/forestry/DISTFRSTMAP.pdf
Best of luck!