Some years back, when they were first required to begin accounting for the miles as a (contingent) liability by the SEC, I recall that one of the airlines made a disclosure in its 10K of the number of outstanding miles, and a possible liability number, based upon the actual marginal cost to transport a person on a US round trip for 25,000 FFmiles (really just the cost of some marginal additional fuel and the meal). I think that the cost disclosed was $25 per trip, or something like that. Seems to me it might have been TWA before the bankruptcy.

But my memory is fuzzy on this one.