First, let me preface this by stating that I am NOT an expert on double rifles. However, I recently purchased my first double following a similar search, and will offer my opinions based on those experiences. You might as well sit down and have a cup of coffee or other suitable beverage, as I have much to say on the subject.
I first started getting seriously interested in purchasing a double rifle about five years ago and have been watching the auction sites and some store prices as time permitted. I had preferences, but retained flexibility regarding caliber and manufacturer.
It appears from your post that you are leaning towards a SxS double rifle in one of the traditional British calibers, so I’ll skip the usual questions about intended use, O/U vs. SxS configuration, caliber type (English vs. metric vs. American modern), etc. with the assumption that you have already considered these matters and know more-or-less what you want. I will also confine my comments to moderately priced rifles, as the high-end items are just plain out of my price range.
My sense is that the overall market is soft. The asking prices for newly manufactured pieces seem to have been decreasing over the past several years, and used specimens seem to linger quite awhile until either the seller reduces the price or a suitable buyer happens along. The blackpowder guns seem to remain on the market longer than those which are nitro-proofed. However, once the price is reduced to an acceptable level the guns seem to get snapped up rather quickly. Unfortunately, I cannot tell you exactly where that threshold lies, but you may be able to gain a general sense of it over time. What I can tell you is that if you miss out, another one will come along shortly.
The Sabatti is a beautiful rifle, and certainly affordably priced, but their reputation has suffered due to shortcuts taken during the process of regulating the barrels. It is my understanding that this problem was limited to a single production run made for one specific buyer, but the stigma remains. That could work in your favor as a purchaser. However when the day comes for you to sell it…….. well maybe not so much. I think the prices you mention for the new vs. used Sabattis should tell you all you need regarding future resale value.
I have handled several Merkel doubles, and found them to be rather stiff and not to fit me well. Of course, we are all built differently, so as they say, your mileage may vary. The factory engraved ones are certainly works of art. But again, I see them sitting on the shelves or on the auction sites for months, sometimes years so the prices may be too high for what the market will bear. (This does provide a good starting point for negotiations though, should the seller be amenable). I would suggest to you that the price you mentioned is a good one for the engraved models, and reasonable for the unadorned version; although I personally would not go much higher.
I attended the big Wanamaker gun show several years ago and had a lengthy conversation with an older gentleman who was selling his collection of double rifles. They included another Sabatti, a Jeffery, and a couple others whose makers I cannot remember right now. The Sabatti was used; the gun was beautiful although the action was somewhat stiff. But the Jeffery – ah, that Jeffery was a pure delight; it shouldered perfectly and fairly danced in my hands! The price was more than I was prepared to pay that day, but the seller and I exchanged information in case our situations changed.
I saw a used Searcy double at a reasonable price, and wanted to contact the manufacturer to inquire about the possibility of fitting and service should that be necessary. Several emails were sent, and several attempts to make contact via telephone (the voicemail didn’t work) but three years later I have yet to hear anything back. Given the size of their operation, I figured that sending it in for potential repairs or modification would be an iffy proposition. Later, I was afforded the opportunity to handle a used boxlock; it was certainly built stout but I just did not care for the way it handled. In fairness, I had already been spoiled by that wonderful Jeffery.
The same shop which had the used Searcy also carried several new and used Chapuis and a Verney-Carron, so I had the opportunity to handle them as well as to learn the proper pronunciation of “Chapuis.” Both brands handled nicely (for me) and the Chapuis were slightly more affordable. So at this point, I had narrowed the field to Chapuis, V-C, and a Jeffery or similar vintage, nitro-proofed English double. I found several Jeffery’s at online auction sites; two of them are still listed and probably will be until the seller reduces his asking price to that magic threshold number (wherever that may lie).
Last year, I was traveling through Oklahoma enroute to a jobsite when I happened to pass near Enid, home to Champlin Arms. On the return trip I arranged for a short visit during which I handled more double rifles than I had in the previous four years combined, all while Mr. Caswell dispensed wisdom, commentary, and advice. He was adamant that the Chapuis was the best modern double rifle for the price, and I have to admit that it is very well-made and fits >me< well. He also noted that the older English (and other) doubles should be expected to hold their value fairly well, as there is a finite universe from which to choose, whereas those brands which are of current manufacture will by definition, be available in new and used versions far into the future. This confirmed a suspicion of mine and reinforced my preference for an older classic British double, should one present itself at a reasonable price.
All this time, I had been periodically checking current prices at the various online sites as my schedule and workload permitted. Asking prices were duly noted, but more informative were the sales prices realized at the various live and internet auctions. If I were to do it right, I should have built a spreadsheet showing the initial offering price, reductions (if any) over time, and the final realized sales price. But I was not that sophisticated, choosing instead to gain a general sense of what was moving and what was not.
My recommendation to you sir, would be to use the “Search” feature on several sites (e.g., Guns International, GunBroker, etc) to determine current starting prices, then search for the realized sales price on the completed auction to determine the price for which it was ultimately sold (or perhaps, remains unsold). I would also suggest that you check online auction houses both large and small (e.g., Rock Island Auction, Morphys, etc.) to see what’s out there and what the gavel price was; remembering to factor-in the buyer’s premium and shipping where applicable. I would also check sites such as Proxibid which will expand the search universe to include some of the smaller auction houses which might offer the occasional double. The best circumstance would be to locate a smaller auction house holding a live (no internet bidding) auction, where the consignor is selling to a crowd which may not have interest in old double rifles, and thus may not be inclined to bid too high. It was at such an auction that I almost (almost!) won a very nice old Joseph Lang double rifle earlier this year. As luck would have it however, there was that one other fellow in the crowd that who also realized what was being offered for sale.
In short, I think that you can find and purchase the double rifle that you want for a reasonable cost. You will need to put in the time and effort to research the going price trends, search out the smaller or specialty auctions, know what you want and what you are willing to pay, maintain flexibility regarding the exact configuration, and be prepared to act when opportunity presents itself. It will require persistence, resourcefulness, and frankly, a bit of luck. But I think that what you want is very do-able.
In particular, JJ Perodeau helps man the Champlin Arms booth. They will have some great guns, but more importantly, JJ knows more about doubles than anyone else that I know in this country - and he is the only person on this side of the Atlantic that I allow to repair or adjust one. Pick his brain.
Now it just needs to taste some buffalo blood 
but I liked what 
