I have access to a black walnut log here in southeast Nebraska. It is about 20" in diameter and 12' long. I am trying to buy it before the guy turns it into firewood, but have no idea what it would be worth in raw log form. According to the Doyle log volume calculator it is about 192 bf.
(Sawing and Drying Forum)
From contributor A:
If it is a yard tree or just a fair log, I would pay $0.80 bdft for it. Just a few months ago I turned down good walnut logs for $500 mbdft ($0.50 bdft). Just did not need any. So a fair price for me would be $153.60 for the log delivered to the mill yard. If you fetch it then they came out pretty well.
Now if the log has no flaws and is not a yard tree full of metal, it could fetch up to $573 for a veneer buyer, but you would have to haul it to them, and it would have to be a very good, sound log.