A new report revealed how much ultra-low-fare carriers depend on ancillary revenue -- and how little Southwest (LUV) - Get Report cares about it.

The report compiled by Stifel airline analyst Joseph DeNardi showed that in general, baggage and change/cancellation fees provide airlines with between 4% and 5% of the average fare.

The outliers are Spirit (SAVE) - Get Report , which derived 28% of its average fare from fees; Allegiant (ALGT) - Get Report , which derived 25% of its average fare from fees; and Southwest, which derived almost nothing from fees.

"As expected, the ULCCs generated fees per passenger at a level meaningfully above the industry average (which is just under $11 per passenger)," DeNardi wrote.

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Aviation consultant Bob Mann added, "Does Spirit think people understand they'll be dinged to that extent even to carry on a bag?" (Mann said he views Allegiant as more of "a vacation travel retailer" that also operates an airline).

As for Southwest, DeNardi wrote that "Southwest's performance relative to the industry was not a surprise at 39 cents per passenger in 12 months ending in the third quarter (and 38 cents in 3Q15) given its lack of change/cancellation fees and no fee for a first or second checked bag.

"Meaningful opportunity for revenue growth exists longer term should it adjust its view on fees." he noted.

In fact, most airline experts seem to believe that Southwest would benefit from adding fees. As Mann asked, "Does Southwest still think it does 5% or more better at the top line to eschew this magnitude of fees?"

However, Southwest CEO Gary Kelly regularly tells analysts that Southwest would lose more than it would gain from fees. In fact, last week the carrier celebrated "'Hidden #FeesDontFly' Day."

"As other carriers tack on outlandish surcharges for what should be considered a normal part of traveling, we reminded folks there's a better, friendlier option," said Bob Jordan, Southwest executive vice president, in a prepared statement.

In general, "ancillary revenue has played a role of increasing importance for airlines since carriers began unbundling the different products and services, and [is] seemingly expected to continue to keep growing into the future," DeNardi wrote.

He noted that bag and change/cancellation fees are high margin and are not subject to the 7.5% tax that is imposed on airline tickets.

In the third quarter, airlines recorded baggage and cancellation/change fee revenue of $1.8 billion, up from about $500 million in the first quarter of 2008, when airlines began to unbundle fees. Still, for now, the fees account for just 3% of total revenue.

Other than Allegiant, Spirit, Southwest and Virgin America, which derived 7% of its average fare from baggage and cancellation/change fees, the remaining six major carriers all derived between 4.3% (United) and 5.4% (American) from fees.

Expressed in terms of fees per passenger in the 12 months ending in the third quarter, fees totaled $21.16 at Spirit, $18.50 at Allegiant, $14.18 at Virgin America, $10.65 at United, $10.15 at American, $9.96 at Delta, $9.56 at Hawaiian, $7.90 at JetBlue, $6.87 at Alaska, and 39 cents at Southwest. Southwest allows up to two free checked bags and does impose size and weight limits on free bags.

This article is commentary by an independent contributor. At the time of publication, the author held no positions in the stocks mentioned.