It might be somewhat valuable but it's worth nothing until you can
find someone to buy it. Most sawyers don't want to deal with singles,
especially harvested from a yard or field, especially if it came from
underground. Too much chance of imbeded metal or rock and not enough
volume to make it worth cutting. I'd check with local turning clubs.
One tip. Cover the exposed raw wood with anchorseal/paint/wax/etc. to
keep it from checking. If it cracks it will be worth significantly
less, if anything. Sorry to have to say that you didn't hit it big. If
you can sell it for turning blanks you might get a couple hundred,
depending on size, quality, and condition. You also might get much
less or nothing at all.