Well, a lot would depend.
Is it really a commercial Riot Gun or military Trench Gun, and so marked?
Or just a cut-off Model 12?
Your gun would appear to be made in 1918, the last year of WWI.
I believe most Model 12 production was still going to the war effort that year, and Trench Guns where still probably a top priority.
A real Trench Gun would have the steel handguard, bayonet lug, and U.S. & flaming bomb ordinance acceptance stamp.
A GI Riot Gun would have the flaming bomb stamp and U.S. markings.
A commercial Riot Gun would have a barrel marked as such.
Any of those three would command a high price. Up to several thousand perhaps, if in good condition and all original
But a commercial Model 12 field grade with a cut-off barrel?
Not so much.
rc