Sarge,
I sold my M-1907 G&K dated 1918 to another Forum member just a few months ago. I don't have my deposit slip handy but I know it was under $100...shipped and insured.
Mine had come to me on a beautiful 1903-A3 Remington and when I sold that rifle, the buyer did not want the leather G&K sling at all, he preferred a standard web sling of which I had several for my Garand's.
My 1907 was also marked W.T.&B. CO which was Westboro Trunk & Bag Company, who was a leather contractor that supplied G & K during WWI era.
These slings were unissued for WWI and remained in Government stock until outbreak of WW II and there were not enough Garand's so the 1903-A3's and 03-A4's got these "NOS" slings issued.
Anyway...I would say you're looking at right around $100 as fair market value.