To compute month to month regular costs, KAYAK normally takes all rates for on a monthly basis over the last 12 months for spherical-excursion flights from London to Atlanta, eliminates the highest 0.1% to account for outliers, then normally takes the median of all values for on a monthly basis. https://www.diigo.com/item/note/aa9i2/gmqx?k=36f2e5831754a5ebc6f1f0a55286a9e6