I looked thru my online Bluebook; there's not a lot of info on pre-WWII Merkel shotguns. The only ones listed are either sidelock models, or a boxlock with sideplates, which is not the case for your gun, near as I can make out.
The modern Merkels run a large gamut of values, and without knowing the model and condition it's impossible to deternmine value.
Greener guns are also available in numerous pricepoints, going up to six figures, and down thru a grand or so.
You best bet would be to find an outfit that deals in high-end shotguns. They would have the expertise to value your shotgun, and probably be able to point you toward a Greener. I haven't been in that game for some time, but you might look and see who's dealing in that sort of gun on gunbroker, for openers. I recall Chadick's in Texas as being a high-end double gun dealer; I believe they're still in play.