Reducing the rotating mass allows the engine to rev faster since its not having to spin so much weight. Its like swapping from a 30 lb. flywheel to a light 5 lb. aluminum one. it is much easier for the engine to rev because it doesn't have to extra weight to spin.
As for the cost, all shops have different shop rates, and some may not be knowledgeable about these types of mods. So you'll just have to shop around.
