First off it should be at least a sentence of 20 years at hard labor for anyone found guilty of cutting up a walnut tree for firewood!!!! Even to suggest such a thing should be considered a felony!
I wish I lived close enough to help you out there, the $200 figure that has been mentioned is about right for the tree you have described but they also should have to clean up the mess for you. I have bought a couple of trees here locally and salvaged one from a couple of dum,,,,,,err,,,well less than real smart folks, who were cutting up a huge very old walnut for firewood that likely would have brought them a couple of thousand had they sold it. I paid $100 for two other trees and gave two fellas $200 ($100 ea) to cut them down and clean up the mess then I salvaged the forks and sold the logs to a lumber yard for most of my money back and I got the forks which is what I was mostly after, if a fork is large and solid it can be worth way more than the log (to the right people, such as me ) ! If you have someone cut that thing DON'T LET THEM RUIN THE CROTCH WOOD BY CUTTING IT TOO SHORT!!!!!! It makes me sick to see some of the otherwise beautiful pieces of walnut crotch RUINED by the logger cutting as close to it as he can, he gains about $10 worth of log and ruins a piece of crotch wood that could otherwise be worth more than the stikin log!
I have a large walnut crotch here drying right now that I gave $200 for and I can't wait to cut it open to see what I have, if there is rot I may not have much and that sometimes happens but usually I can find a few pieces that are the equal of wood blanks I see on Ebay for as much as $1000 or even more. If you have that tree cut then have the logger leave at least a foot of wood below where the bark starts to split. You might be surprised at how much interest you would get listing that crotch on Craigslist.