There is no reason it shouldn't yeild the same value as the game, but there is nothing to say it has to either. I view DLC as esentialy a 'new' game you are purchasing. The DLC and the game are not one entity, they are completely different products. (In a transactions sense)

It is like saying if Company A makes and awesome game that you play for 200 hours, you will never buy a another game from them that isn't worth 200 hours or over. Using your philosophy you will shut yourself out of the new game market fairly quickly.