It was posted in InsideFlyer once how much each airline valued the miles against their accounting records. They valued them in 25k blocks (for a domestic RT - the most commonly redeemed award) at between $15 and $25.

But they sell them at a minimum of like 2.5 cents per mile.

And the members seem to value them anywhere from 0 to 10 cents a mile - an argument that has gone on forever.