Sooner or later, 240 owners will move up to the 350z, and the cycle will continue. If we woke up tomorrow and every car manufacturer stopped making RWD cars THEN I would say a 240 may increase in value a grand or two if it was stock and clean.
Think about it this way: You've just got a decent paying job, and you're looking to buy a car. You remember your mad tyte drifta days and consider looking at old 240SXs. You find a few, all with dented body panels, and all with hack job swaps. For the same price you find a RWD coupe that is brand new, comes with more power, and would be more reliable. Which would you go with?