Cost is generally a more holistic concept than price. If you use money (or any resource) to do X, then you can't use it to do Y. So if your money supply is limited, every purchase is more costly to you because of what you must forego to make it. For the same reason, a 10k raise when you make 1M is not worth nearly as much as a 10k raise when you make 30k. It's the same 10k dollars, but has very different worth depending on who's dollars they are.