Dutch,
Great build, sure your son is delighted!! I hate doing large glue ups. Have to get thicker lumber so you can make them flat and square. Looks like a great sander your friend has, a good guy to know. The wood grain looks very nice.
Just checked my usual wood store and $14.25 for 6/4 $15.00 for 8/4. But you bought enough that I would have gone to Hood Distributing and the $12.50 that Lazyman mentioned would be in the ball park. With Hood you have to buy complete boards and I usually have a $50 piece of lumber left over.
Our son's wife wants me to make her a mid-century modern coffee table, also walnut, but surely not as much lumber.