Press 
The Financial Times pegs the value of Delta’s loyalty program at a whopping $26 billion, American Airlines at $24 billion, and United at $20 billion. All of these valuations are comfortably above the market capitalization of the airlines themselves — Delta is worth $19 billion, American $6 billion, and United $10 billion.
Source: Why the Future of Delta and American Airlines May Depend on Frequent Flyer Miles | Marker
