[Alan Roehrich]

You buy the lightweight stuff when you are building to begin with, or you need new parts. Then it is worth the money. If you're replacing perfectly good reasonably light stuff that is working now, with high dollar trick lightweight stuff, most often it is a waste of money better spent elsewhere.

The lower the power of the engine, the more it has to gain from light parts, as it has no "surplus" power.

Too many people go spend meg dollars on trick stuff long before they've mastered the basics, and maximized the rest of their combination. These are the people who have rarely if ever rented a track or spent days at test and tune sessions.

To the original question, the only way to know what it is worth is to test back to back on a given combination, with serious A-B-A testing. Most people who spend the time and money to do that are understandably quite reluctant to share that hard learned data with anyone, save a few people that share with them.

Some of the very fastest cars have none of the trick stuff in them, not even in the engine. One engine builder responsible for the legendary performance of a certain Stock Eliminator car once told me, while we were discussing dyno parts, that his "shop was filled with camshafts and headers the engine and car did not like", and "the secret to it being fast was not so much knowing what the car liked, but more a matter of learning everything it didn't like".