Question about the SkyWish program. This is accounting related.

My client receives donations of miles via Delta's SkyWish program. They have to recognize revenue when they receive the contributions. My delimma is what value to assign to each mile donated.

You can buy SkyMiles for 0.0275 per mile plus taxes and a processing fee. However, I am trying to determine the actual 'cost' of each mile.

Is there any publication that will give me the average length of a Delta flight in miles and the average cost per mile of a traveling passenger? (i.e. average flight in miles multiplied times the average cost per mile gives me the cost per flight and I can value the miles from there).