Just what, exactly, is a fair price?

A local merchant does a radio ad, promising “a good product at a fair price”.
Now, I figure most people know that this is nothing but “market-speak” designed to manipulate those who imagine that everything is too expensive or a rip-off.
But I also suspect that to some people, the phrase actually has meaning, as if something actually makes a price “fair”.
Who or what makes a price “fair”.