Margen
Every euro has two prices: the one you pay at the register and the one you forfeit by not using it on something else. The second is called opportunity cost, it never appears on the receipt, and it's almost always the higher of the two.
Here's an example with round numbers: replacing a high-end phone every two years can run you about 600 € a year. Those same 600 € a year, invested at a 7 % average return, come to roughly 24,600 € after twenty years. The real price of the phone wasn't 1,200 €: it was everything those euros could have become.
This isn't about never spending —every expense gives up something— but about spending while knowing what you're giving up. The rich question isn't "how much does it cost?" but "compared to what?"