What is the value of a 1918 Browning military rifle?

You seem to be referring to the Browning Automatic Rifle Model of 1918. That is a selective fire rifle, and requires registration under the National Firearms Act. Any legitimate, registered BAR would be $20,000 or more. Any BAR that is not registered is worth up to ten years in the penitentiary and a $10,000 fine. Ohio Gun Works is making a semi-auto civilian legal BAR with a retail of $5500.