You figure out how much weight you want to save off your current rig.
You go to the ATM, you withdraw that same amount of weight in $20 bills (i.e., saving 100 grams of weight off your bike requires you to withdraw 100 grams of currency).
You then hand that stack of money to your local bike shop.
Your bike then gets lighter.
(btw, I did not invent this formula, I just finally figured out that it is true. slow learner)