If you spend $40 to fill up your car (but you were willing to pay as high as $120 to fill up your car), then $80 of value was created out of nothingness. You got $120 of value (given that you _would have_ filled up your car at a much higher price anyway), but only paid $40 to get it.
This is fundamental to the theory of capitalism. The trade isn't grounded at the paid price (ie: $40 in this case), the trade is grounded in the price _YOU WOULD HAVE PAID FOR_ vs what you actually paid.
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If you are paying above the price what is comfortable for you, then you must stop paying for the system to work.
Now obviously: capitalism stops working in cases where you'd pay any amount of money (because now the opponent would choose any price and force you to pay). This happens in monopolies and health care. (There's no limit to the price you'd pay to stay alive).
However, I still posit that in the vast majority of cases, that capitalism works. Negotiating for a lower price is assumed to happen on both parties.