The market determines the price of everything, just like now. Everything is supply and demand. When you see something is expensive, it might be because that particular thing is scarce. When you see that everything is getting more expensive, it's because the money itself is getting LESS scarce.
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This 👆
Plus, deflation not only compared to fiat, because the more people use bitcoin, the higher its price. Which is independent from the printing of fiat.