I had to add a bit in here as I buy and remove a lot of walnut during the course of a year.
Firstly worth, a lot of what I get is free, or at least if it is suitable quality and covers the cost of removal or felling, so whilst I might not pay the owner when still standing they have it removed for free or at a lower cost, so free is not the right word really. I have also paid £20 on measure for the right butt. What is the right butt? no need to do that one really as everyone knows about sap rings, shake, colour etc. Is it worth sending in a artic load to a mill? Only if the quality is good, you can't base this on the number of logs. I have just finished selectively extracting 7butts from almost 80 in a old walnut grove of around 90years of age which is not that old really. The rest are worth more as firewood than timber, large sap rings, poor form etc.
Gambling on standing walnut, the only gamble is on shake really, that can be accessed from experience of the site and the exposure, the ground it stands on etc, the colour, sap ring etc is always inspected first so I know what I am getting, some varieties make for good timber some don't, but by inspecting each log it will always tell it's own story.
Lastly root balls? One day the myth of making thousands selling stocks will end hopefully, but then again I have numerous logs with rootballs on, one large log even has some nice burrs attached all over it, will I cut it for stocks? no, complete waste of time, the market is flooded by east europeans supplying this very small limited market for high end stocks which are produced alongside the very average ones sold to large gun houses producing thousands of stocks each year. We take the root balls only on very promising butt's with good stripe, only because it can produce stunning timber which is a lot easier to sell than some piece of sappy grey washed out stock.
That's my take on it and I deal with inspecting walnut almost on a weekly basis.
You can see a few bits and bobs of these on our website primetimber in the news section