Although I have seen 2 and 2.5 cents per mile as benchmarks, I disagree. Mileage awards, for the most part, cannot be considered as valuable as what it would cost to buy a ticket because:

1. Often free (Award) seats are not available when you want them.
2. More blackout dates.
3. You do not earn miles with free ticket.
4. You cannot easily get upgraded with free ticket.
5. You often cannot get moved to another airline flight if flights delayed/cancelled.

Therefore if 25K miles for a flight that costs $400 (very few places in US I cannot go for $400) -- one does not get the same benefits as if one purchased the ticket.