OK, but that would suggest quite a range of possible 'fair' return.

So, let's suppose that fashion suddenly turns against hats, and all these hat makers (say sole traders and co-operatives) have to slash their prices or, in desperation take up another trade.

Is the low price a fair one? Even though it causes desperation? Or if not, what would you like to see done about it? Is it fair that the similarly-skilled wig maker is suddenly earning a lot more than the hat maker?

What I'm getting at is that I'm not convinced that 'fair' price or return is a useful concept.