It gets even more complicated (and more controversial) if you're talking about a class of service or a flight you could not afford otherwise. Just because you redeemed for a business class flight which would have cost $5000 cash, but you wouldn't have paid $5000 cash for that flight if you couldn't have redeemed miles, is it legitimate to now value the miles as now being worth $5000? If you would have flown in economy class instead, perhaps it's clear that it's maybe not worth that much, and that maybe the value is somewhere between the economy class flight you would have bought with cash and the actual business class flight you booked. But what if you would not have flown at all (because you're tall and you can't bear flying a superlong flight in economy)? So if the business class redemption hadn't been available at all, you simply wouldn't have taken that trip. Well, in that case, it's not at all clear how you should exactly value the miles, because they're no cash comparison point (of what you would have bought).

So the value of miles used "aspirationally" is a lot less clear than the miles used very "practically".

Some examples: A $5000 business class round trip may cost only 100k miles, which seems like a 5c a mile redemption. And a $11000 first class round trip may cost only 150k miles, which seems like a 7.3c a mile redemption, but even fewer people who have actually paid $11000 and would never have considered at least a business class flight instead, so the 7.3c mile valuation is even more obviously suspect.

Also complicating analysis: The time at which an award redemption may be available (either 10 months out or 3 days out in some cases) does not correlate to the time when the lowest cash prices may be available (6 to 8 weeks out in many cases, or else during sales whenever they happen). So do you compare against the cash price it would have been when you booked with miles, or do you compare against the cash price it would have been when you would have booked with cash (which would certainly not likely be 10 months out or 3 days out)?