I think what LPR is saying is that its stupid to say "I redeemed my 50K miles for a $5000 ticket so I got 10 cents per mile", if you could have just bought 50K miles for $1500. Since you are dealing with all the restrictions of using miles they can inherently not be more valuable than the cost of buying the miles. It might be rational to spend the $5000 on the ticket because you earn miles, or its tax deductible, or there are no award seats available, but you simply cannot say that the miles were worth $5000 if you can buy the miles for $1500.

Another example might be my Panasonic plasma TV. If might be worth $5000 to me. I enjoy it that much and if that's how much all tv's costed I might be willing to pay that. But I do not say its worth $5000 because that's how much I would be willing to spend, its worth at most $1000 because that's how much it would cost me to replace it.

Put most simply:

The value of something is the lesser of the value derived from it or the replacement cost.

If 50K miles gets you a flight that cost $5000 retail but you would have only spent $1000 and 50K miles cost $1500 then the miles are worth 2 cents each. If instead 50K miles only cost $750 then the value of the miles is only worth 1.5 cents.

I also question even 2 cents as the value of a mile because I have seen plenty of opportunities to buy them for less. If everybody is valuing them at 2 cents then there should be a mass rush to buy when they sell for 1 or 1.5 cents.