Assuming you are looking at a 12 bore SxS, USA retail, 1890ish to 1960ish, I can give you a ballpark value based on three factors from my market model. The premise for this market model is that guns are basically valued on three factors; Brand Value (BV) of the maker’s name, Original Quality (OQ) grade of the individual gun, and Current Condition (CC) of the individual gun. Note that despite the oft repeated question, "Did Mortimer Snerd make good guns?" OQ was a function of price point. All makers made their guns to price. Most makers offered a wide range of quality grades at price points. However, some makers achieved more brand value than did others - this value being independent of the quality of two identically "good" guns.
A&N achieved Brand Value level three (BV3) - they will generally retail for half the price of similar BV1 (Boss, H&H, Purdey, & Woodward) guns and two thirds of the price of similar BV2 guns. Original Quality grade can be determined from existing catalogues or experience. In this case, if you can provide the serial number, you can most likely get the exact cost, selling price, date of sale, and maker by contacting the University of Glasgow Archives where the A&N books are kept and made available. Current Condition level is the third factor. Is the gun "Pristine" = CC1, "Slightly Used" = CC2, "Significantly Used" = CC3, "Heavily Used but not Abused" = CC4, "Restored or in need of finish restoration" = CC5? There are are four more CC levels down to CC9 = "Wall Hanger," but you are not likely looking at one in this range.
Also, you can "reverse engineer" the price by looking up the asking price relative to OQ or CC for the BV3 level and asking yourself if the OQ or CC needed to make the price makes sense. Your $6,000 will be in range of a A&N SLE as follows: best gun (OQ1 - pretty unlikely) in CC5, "A" grade (OQ2) in CC4, "B" grade (OQ3) in CC3, "C" grade (OQ4) in CC2, and a "D" grade (OQ5 - also pretty unlikely) in CC1.
Post photos of the gun or a good description and we can refine the price range. Odds on this gun will turn out to be a W&S Screw Grip at one of the several finish levels in their catalogues.
Let me know if you have questions.
