Good question. I think UA carries miles on their books as a liability at $0.014 per mile, which is the average cost of redemption including UA flights and payments to partners excluding breakage (unused miles). I am guessing the cost for partner flights is about the same as for their own flights. According to public statements filed by DL, they carry miles on their books at $0.0054 per mile. I think this may be based on the sales price of miles to Amex rather than redemption cost (but they should be roughly equivalent), which implies that a SM mile is worth about 40% of what an MP mile is worth.