Don't know where the trees are located, but here's a link to a very good fact sheet about the value of "urban" (in other words, "not in the middle of a forest") Black Walnut trees:
http://ohioline.osu.edu/for-fact/0044.html
Relevant information needed to even guess: are you measuring the diameter at breast height? How tall is the useable trunk section of each tree (straight trunk before any branches or forks)? How free of defects are the trunks?
The gist: they are probably not worth anything, money-wise... Even if the useable trunk sections are the bare minimum 8', and absolutely perfect, Black Walnut has a relatively large sapwood layer (3" or more). So even in a perfect 16" DBH tree, you are already down to 10" diameter heartwood at most. Remove the pith and you lose at least another inch, even if the pith follows a perfectly straight path. Hopefully you see what I'm getting at...
The yield would be tiny, even if the trees are perfect. Mills (at least where I live) won't take anything less than 20" diameter, 8' long with no structural defects. And unless there is something incredible about the tree, they won't take an urban tree at all, because it is almost guaranteed to have metal embedded in it.
But there's good news if you look at the "value" of the trees in a different way.... Why not see if there are any woodworkers, especially turners, in your area you could give the wood to? Some might even be equipped to take them down for you in exchange for the trunk wood. Any decent turner would probably turn a bowl or hollow form for you, in appreciation, without even being asked. You would know that your trees would be used in fine work that will last many years and be used and/or appreciated.
In my experience, people tend to overestimate the value of their trees that happen to be one of the "marquee" species, and end up disappointed and frustrated. If they shift from thinking of the trees as dollars to thinking of them as raw material for a local artisan, they are more satisfied, and usually end up with a beautiful natural-edge bowl to set on the mantle and admire.
Of course, this is all just my humble opinion. Your trees might very well be covered in burls, and you could end up getting $5,000 per tree from a veneer buyer. And if that's the case, I'd be honestly glad I was wrong and congratulate you.
Please post follow ups. I'm curious to know how it goes. Good luck!