Not owning a double rifle myself......yet, I'll state a range of $7 - 10K for a base model used double in very good condition.

The problem lies in the description of "base model" as it comes in all flavors from the Merkel/Krieghoff range up to the Holland & Holland/Westley Richards range. They all have "base models".

You should be able to get one for the former models I mentioned within that price range. I'm headed to the Vintage Cup in Maryland early tomorrow morning and can take a look for you and follow up when I return.