FWIW, I just found this and thought it was somewhat interesting. Miles are now so highly commoditized that they are literally listed a a genuine form of currency:
Privately-issued currencies
Several large companies issue points to their customers, to be redeemed for products and services produced by that company. Often, a network of companies will join to share in the offering and redemption of points. While these can hardly be considered stable currency systems, they present many of the same features as "legitimate" currency: they are a store of value, issued in discrete units; they are controlled by a central issuing authority; and they have varying rates of exchange with other forms of currency. For example, frequent flyer miles can be bought using U.S. dollars...
Frequent Flyer Mile: The most commonly-known points systems are the frequent flyer miles issued by major airlines. The first such system was issued by American Airlines. Other customer loyalty incentives have followed this model, including points systems offered by soft drink manufacturers such as Pepsi. Subway tokens, issued by city transit authorities, can be considered a highly specialized form of currency...
http://encyclopedia.laborlawtalk.com/Monetary%20unit
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Also, FWIW...
... Don't overpay for miles..
...There are two ways to assess this. The first is to figure out the cost per mile. The industry standard values a mile at two cents...
...If it takes you four years to accumulate 25,000 miles from your credit card, enough miles for a free ticket, and your annual fee is $50, you paid $200 for that ticket. As long as you book an award ticket that would have cost more than $200, you've gotten a good deal. If the cost in annual fees ends up higher than the dollar value of your award flight, you are not making the best use of your credit card...
...Take advantage of promotions
Sometimes airlines offer special deals that are only available to affiliated cardholders. For instance, Delta offers "Always Double Miles," an ongoing promotion that allows SkyMiles cardholders to earn double miles for groceries, stamps, Delta flights and other items. On top of that, American Express will from time to time offer double miles on every purchase for SkyMiles cardholders. It's good to sign up for these promotions and try to time big purchases accordingly, so you can maximize your miles.
In addition, certain airlines, such as American, offer award discounts on certain routes to cardholders. These offers let you book free tickets for fewer miles, saving you both money and miles. The best way to find out about these promotions is to receive the credit card's targeted mailing and e-mail newsletters. You also may want to check the credit card's website once or twice per month, as well...
http://www.usatoday.com/travel/deals...8-column_x.htm