Greetings Vennie,
Unfortunately, some questions just don't have a good, clear answer, and this just so happens to be one of those questions. There are a lot of variables that come into play regarding tree value: diameter, defect, merchantable log length, volume, accessibility, etc. Not to mention, you did not provide me with a DBH (diameter at breast height) measurement of the tree. We could have at least estimated this tree's volume if DBH and merchantable length to a minimum top diameter were known and provided.
Most 30 year-old black walnut trees are going to have low saleable value because these trees are simply too young and too small to contain any significant volume for lumber or veneer.
I realize this isn't the answer you were looking for; however, it's the best answer I can provide you based on the information you furnished in your question.