Which airlines have the most affordable premium economy seats? As a rule, low-cost airlines with premium offerings will undercut established legacy carriers, like Delta Air Lines, United Airlines, and American Airlines, but this is not a perfect rule. When dealing with averages, it is important to keep in mind that an airline that typically has higher-than-average fares may have deeply discounted fares, making it the cheapest on that day. The reverse can also be true, with low-cost carriers having the most expensive fares on some days.