Greetings Martin and thank you for your question.
Essentially, you are looking for a professional appraisal of your timber. More specifically, you are looking to determine your timber "basis", which is the value of your timber upon date of purchase or inheritance less the value of the land, buildings/home, or improvements. Included in this basis will include the value of those walnut trees (you can ask the forester to value each of them separately to satisfy your curiosity and investment objectives).
Timber appraisals are common and most professional consulting foresters either charge a flat fee for a given acreage or charge on an hourly basis. I guess what I'm trying to say is, "find a consulting forester who will only charge a flat appraisal fee or an appraisal fee on an hourly basis." Since you are not interested in selling your timber at this time, you don't need to pay a commission to the consulting forester.
Some numbers of seen come across my desk in years past included $50-60/hour plus mileage. I've also seen $200 flat fee charges plus mileage. Obviously, the more acreage you have the more time it is going to take the consulting forester to provide a statistically accurate and honest assessment of the timber value. These figures are from Illinois and should only be used for informational purposes.
Please call your local PADCNR forester for a list of professional consulting foresters who are capable of providing these services for you.
Best of luck!