Why is... Une diminution des prix serait positive....is correct? And why ....Une diminution de prix serait positive.... is incorrect. I would think that later should be correct because clearly "de' is all about Une diminution. Why are we using plural here?
Actually, both should be correct. Une diminution de prix means A price reduction (one price = one object) because de prix is singular. Une diminution des prix means A price reduction (many prices = many objects) because des prix = de + les prix is plural.