Classic answer - it depends.

Depending on location walnut can bring $5 or more (sometimes a lot more) per
board foot. A tree with a good, long straight trunk, while rather rare,
can be worth a lot of money.

Check local sawyers, hardwood dealers, etc. You might get someone
interested in taking a look at it, My in-laws had a huge oak taken out of
their back yard a year or so ago. The tree produced a lot of lumber and the
fellow did the job for about 1/3 of the lumber. They still have a lot of
air-drying regular and quarter sawn oak.