Greetings Shane:
It is practically impossible for me to value or appraise this tree without seeing it first-hand. Therefore, I would strongly encourage you to seek professional forestry assistance from a local state forester or a local professional consulting forester in order to value/appraise this black walnut tree and to find a potential timber buyer for this tree. In all honesty, there are a lot of woodworkers out there who want trees just like this--you just have to know how to find them and market your tree!
I would encourage you to start here, with the Nebraska Forest Service:
http://www.nfs.unl.edu/NFSdistricts.asp
Best of luck to you!