To determine if a purchase is worth it, calculate the : 4 Times It Makes Sense to Buy Airline Miles - NerdWallet

: If you are short by a few thousand miles for a specific, high-value redemption, buying a small amount to bridge the gap is often worth the premium.

: It is almost never worth buying miles for coach/economy seats, as the cash fare is typically lower than the cost of the miles needed.

: Never buy miles without a specific trip in mind. Airlines frequently devalue their miles (increase the number required for a flight) without notice.