The "cheap" in buying miles usually comes from timing and specific circumstances: 4 Times It Makes Sense to Buy Airline Miles - NerdWallet
Buying frequent flyer miles "cheap" is generally a strategic move used to bridge a gap for a specific reward flight rather than a primary way to accumulate rewards. At full price, airlines typically sell miles for , which often exceeds the actual value you get when redeeming them. Strategies to Buy Miles Cheaply