Greetings Jerry:
Seeing that black walnut prices are still very hot in the Midwest, may I suggest the following options:
(1) contact a local professional consulting forester to assist you in appraising the trees in question [recommended option] http://dnr.state.il.us/conservation/forestry/CONSULTING_FORESTERS.pdf.
(2) contact other licensed timber buyers in your area who may be interested in your trees
http://dnr.state.il.us/admin/systems/Pdfs/TimberBuyers.pdf
(3) if you know how to calculate tree volume, then you can approximate the tree's value by looking on my Timber Blog http://web.extension.uiuc.edu/forestry/blogs/eb94/
or by looking on our state's timber price bulletin http://web.extension.uiuc.edu/forestry/il_timber_prices/index.html
Now, if you don't know how to calculate tree volume, then I suggest you contact a professional consulting forester to assist you.
Best of luck!