There are an estimated 3.5 trillion miles outstanding among all frequent flyer programs.

I pulled this off of best fares web site. 3.5 trillion outstanding miles! Divide that 25000 for a flight to get 140 million flights and multiply that by say, conservatively, 300 dollars per flight, to get 42 billion dollars. Obiously, people blow more than 25000 miles on average per flight, but even if it was 50000, it would still be 21 billion in outstanding "dollars". That's quite a bit. Just my $0.02.