High prices are based on rarity and demand. With a generational turn over it becomes apparent how plentiful ordinary field guns are, thus falling prices.
Some guns are out of fashion, as stated above.Semi-autos rule for waterfowl, and pumps are less common than they used to be on the prairie.
Choice Browning Superposed still generate a lot of interest, particularly 20 gauge.
Not much surprising about lack of demand for common guns.