It assumes the following logic: If it is more important to you than to someone else, you'd be willing to pay more for it.
The fairness of this then is inversely proportionate to the gap between the rich and the poor. In absence of any regulation or mitigation this results in effect that the rich people are more important than the poor people.
I wish I had mod points right now!
Many economics-oriented people don't seem to want to think about that notion about the gap when it comes to fairness.