uhm, as an economist by training I would suggest:
NOT to look at
-what people paid for miles AND a flight (i.e., when you fly and get miles for doing so), because that confounds the value of the flight and the miles
-what you can "get" for the miles, especially if these values are inflated (e.g., a $20k F ticket)

Instead, I WOULD look at
-the market prices for miles; it should most accurately reflect what "value" (monetary, and I suppose that's what you want to get at) miles have to people. It doesn't matter what they want to do with them, does it (in fact, people like myself just like to see miles piling up in an account - so be it!).
More concretely, one could look at
-price for miles/certs at ebay
-price for miles purchased on airline websites (although it would be nice to know what the actual demand is)

Of course, it is complicating that the value of a mile may differ across people and even within people. Some might wonder why someone pays $25 for 1000 miles on ebay - but if these miles were missing to get a free ticket, they are "worth" it. Still, I think looking at (market) prices is the most viable way to find out what miles are "worth".