Greetings Angie,
As I've mentioned before throughout this forum, I receive this exact question almost daily. Unfortunately, it is practically impossible to appraise or value standing trees or logs via this forum. Why you ask?
The answer is rather simple...timber values vary widely due to species, merchantability, quantity, and quality (i.e., grade). Simply providing the dimensions of a standing tree or log is not adequate to answer your burning question, "How much is my black walnut tree(s) worth?" Heck, your tree might have a bunch of defects or metal in it, rendering it almost worthless as a sawlog. However, your tree might have a perfectly straight, defect-free 8-ft log containing 242 board feet of FAS lumber or veneer.
Therefore, my recommendation is quite simple: contact a local professional forester or a custom sawmill owner to assist you with an on-site evaluation and assessment.
Best of luck to you and Happy New Year!