The cost to upgrade to Delta Comfort+ is not a fixed price; instead, it utilizes dynamic, date-based pricing that depends on the route, flight duration, and current demand. Generally, for domestic U.S. flights, you might see upgrade offers ranging from $15 to $150 per segment. For long-haul international routes, the price can climb significantly, sometimes between $100 and $500 or more, depending on how close the flight is to capacity. Comfort+ provides up to 3 inches of extra legroom, dedicated overhead bin space, and "premium" snacks and alcoholic beverages. It is often more cost-effective to book Comfort+ at the time of initial purchase rather than waiting to upgrade later, as the fare difference is sometimes lower than the post-booking upgrade fee. If you have Delta Medallion status, you may be eligible for complimentary upgrades to Comfort+ shortly after booking (for Diamond and Platinum members) or closer to departure (for Gold and Silver members), making it one of the most popular perks for frequent flyers.