Even when there isn't economic disparity between the two tenants, the amount of sacrifice represented by $100 may well be unequal.
Fortunately, this system involves everybody listing exactly how much they'd be willing to pay for each room so that nobody ends up sacrificing an amount that they're unhappy with for the room they end up living in. So even if they're sacrificing unequal amounts, they're getting a result they're happy with.
Fair in the sense that nobody is forced to pay more than they want to / can. Unfair in the sense that one person has more power than the other, almost definitely because of their family history, not because of anything they have done specifically to deserve that extra freedom and power.
My point is basically that our society allows an unjustly large economic disparity, and to call this a "fair" method of divvying up rent ignores that nothing involving money can be truly fair in our society.
