I'm certain that the true value of a mile is "it depends." It's different for different people in different situations, and even for the same person at different times.

Here's one interesting thread, from the Mileage Run Forum: http://www.flyertalk.com/forum/showthread.php?t=276409 Although it's from 2003 and hence possibly slightly outdated, the kinds of thinking and information people take into consideration are still valid.