Value will depend on the quantity and quality.

Quantity is easy to establish. Rough lumber is priced by the board-foot, which is 144 cubic inches. A plank 12" wide by 1" thick by 10 feet long is 10 board feet. An otherwise similar plank that is 2" thick is 20 board feet.

Thicker lumber is more valuable, but harder to sell, than thinner lumber because thick wood is harder to dry properly.

You'll need to get an idea of the lumber's grade. Wood with nice, straight grain* and no knots is more valuable than wood that's full of defects.

*Some wood, with grain that's very wavy and dramatic-looking can be especially valuable.

Was the wood ever kiln-dried? Kiln-dried lumber is more valuable than air-dried lumber.

How has the lumber been stored? If it's been in a basement or shed that isn't climate-controlled, then it will be too damp to be used immediately so its value will be substantially lower.

Boards wider than 6-8 inches also get a premium if they are clear.

You can look at any number of online sellers of hardwoods, and then call some local hardwood dealers to get an idea of price.

Walnut is a relatively expensive wood.

In addition to the above, some people prefer the subtle color nuances of air-dried, non-steamed walnut. But then there will be more waste trimming off the sapwood.

I would not expect to pay anything like lumber store prices ($6-10 bf) for walnut sitting around for years, stored in barn. I'd only be interested if it was 10-20% of that price. I've worked a fair amount of harvested lumber and know there is a lot of waste in it.

But if you bought this graded, from a dealer, and just never got around to using it, then I would pay more for it.

If it is not full of cracks or checks then it should be saleable. Is it FAS or 1 or 2 common? Sounds like it would be kiln dried, but if it is air dried with not too much sap wood then that is a plus. How much do you have and what length, width and thickness? If all is good, I would say about two to three dollars a board foot, but watch out for low lifes comming around trying to rip you off.