Doesn't this apply to anything you can buy from multiple places? Milk may be $1.50 from a corner shop but $2 from a supermarket. What price is the "real" one? T

Doesn't this apply to anything you can buy from multiple places? Milk may be $1.50 from a corner shop but $2 from a supermarket. What price is the "real" one? Taking a mean average based on trade volume seems to be the most logical way to value it. I guess as good becomes more liquid (like USD and gold) then the gap gets reduced.