Originally Posted by
Happy
The airline which miles you used to claim the award would pay the partner airline you will fly on, an agreed-upon price for the seat you redeemed. It is a reciprocote agreement among partners who signed into an alliance. Needless to say, the formula to calculate such cost is a highly guarded business secret.
And in a perfect world, each airline would book a similar number of seats on each other's partners flights so things balance out. Again, how that all is calculated is unknown.