"Even with the fees, if you get one of their $9 fares minus a $24-off promo code, they are a discount airline," he said. "And their Big Front seat (sort of business class) is a real bargain. Whether they're a good airline or not is up to question."
Spirit was followed in the fees-as-percent-of-revenues list by AirTran (10.2 percent), Delta (9.5 percent), Allegiant (8.8 percent), US Airways (8.6 percent), Virgin America (8.2 percent) and Frontier (6.6 percent).
Northwest Airlines, which is now part of Delta, came in at 5.9 percent. Southwest and JetBlue round out the top ten, with 5.8 percent of revenues coming from fees.
United, Continental and American, which all took in millions in fees but rely on long-haul international ticket sales for the greatest profit margins, did not make the top-ten list.
Southwest doesn't charge for checked bags, phone reservation or changing tickets but still took in millions of dollars from assorted fees -- including the $75 it charges each way to bring your pet onboard.
"Southwest Airlines has been smart enough to not pit themselves against their own customers," said Jeff Pecor, of travel site Yapta.com. "In fact, they're using the 'me versus the airline' issue to its competitive advantage by marketing 'no baggage fees' quite heavily. The airline has managed to find ancillary revenue in areas other than baggage fees, so it begs to be asked: Why can't other low-cost carriers?"