If you are referring to this model http://www.doublegunshop.com/merkel_47sl.htm
...it is a very fine gun. It is the entry level sidelock....simpler scroll engraving on bright lock plates. For the most part these guns were built with grade 1-2 wood...a gun with better wood would be worth more. Those built specifically for North America usually came with the better wood. They are very rugged guns...not as elegant as many offerings from Spain or Italy, but built for a lot of hard use. A lightly used gun will still be stiff...it takes a lot of rounds to break in these guns usually. Should be in the 6lb 14 oz to 7lb 2 oz weight range...not as trim as Spanish guns. Only think I don't care for is that often the ejectors throw shells across the yard!!! but that can be tuned.
I have a 147sl in 16 gauge (same mechanicals, just better engraving and wood) made in 1976 and it is unbreakable. I would think in top condition, your gun should be in the $4K range...sub gauges would be a bit more....price adjusted for engraving, wood, single trigger, overall condition, etc.