The price of anything is the amount of life you exchange for it.

Money is a stand-in for hours already spent, so the real cost of a purchase is the portion of your life it took to earn. Seen that way some things are obviously worth it and others turn out to be expensive in a currency you cannot replace. The same accounting applies to commitments taken on for reasons that no longer hold.

This is not much use where the spending is not optional, and plenty of it is not. Where you do have a choice, convert the price into hours of your work before deciding. It changes surprisingly few purchases, and it changes those completely.