With all due respect to those who favour mathematics over aesthetics, I have different questions for the OP.
Do you like the gun in your hands? Does it feel right? Does it fit you and is the stock in satisfacory condition to live with or be refinished by skilled hands. Are there any flaws that will niggle at you later?
Once you have evaluated the aesthetics of the gun to your tastes, evaluate the technical aspects.
Do your due diligence regarding bore diameters vs proof. The British are/were over fond of lapping out pits. Has the gun been buffed over the engraving?
Only after all the above should you go on to establish a price.
A fair price can be higher or lower than formulaic analysis indicates, based on how much the gun calls to you.
You sound like you want a using gun, and not a short term fling with a floozie. Flaws never get better with time. Buy something that you really, really like.
