There's always been this idea that black walnut is extremely costly .While perhaps it is it isn't worth any where near 15 -20 thousand a tree .
I just looked up stumpage prices in Ohio which is about a year old and prime lists for 1500-2 thou a thousand board feet .Now that's twice ,3 times what prime white oak would be and figured on a goodly amount of timber not just one tree . I'd guess locally those prices would be 2/3 of the state average .Just a few trees or less than prime you could get about as much out of firewood than selling to a mill that acts like they are doing you a favor just to haul off the logs .
Remember now the midwest ,Ohio ,Indiana ,Iowa ,etc is called the "walnut " belt and that tree is more common in this area than the rest of the country .Black cherry is common too .