Too many variables.
Price depends on true condition, which requires an inspection of the specific gun. One person's excellent is another's Very Good, another's Excellent Plus, and another's 95%.
This is all subjective. One person may rate a gun with a minty finish and a worn barrel higher than a gun with a slightly worn finish but in perfect time.
Where it's at. In some areas, Pythons are popular and bring high prices. Other areas, people are more concerned about price. There, Python's are seen as too expensive, and don't sell, thus lower prices.
The gunshop selling it may attempt to sell it for more than the going local price, or it may be low because they want to move it.
Some areas, nickel guns, especially Python's don't sell. There's an old rumor (incorrect), that Colt plated Python's that had a defect in the polish job, so they weren't as good as a blued model.
Nickel guns are good sellers in other areas.
Go down to the local book store and get an average price from the various gun value books like The Blue Book and The Standard Catalog. This will give you a idea about price. You can then judge value based on the average.
Guns are a commodity. Ice water sells real well in Hell, but not in the Arctic.
Bottom line: you have to judge price on THAT specific gun, and how bad you want it.