Source: Allegiant Air

You've heard of highway robbery; now welcome to the fast-growing trend of skyway robbery. Certain small airlines in particular are mastering the art of luring passages with cheap upfront fares and gauging them with expensive fees later. Allegiant Travel Company (NASDAQ: ALGT) is quickly catching on and making its shareholders rich in the process. 

More moola per mile
Allegiant reported fiscal-fourth-quarter results on Jan. 29. Revenue increased 7% to $238.5 million. Operating income jumped 19.3% to $30.3 million. Net income climbed 18.2% to $17.5 million, and diluted earnings per share leaped 23.7% to $0.94. It was the 44th quarter in a row of profitable results.

CEO Maurice J. Gallagher pointed out that it was the second full year in a row of growing both operating margins and earnings before interest, taxes, depreciation, and amortization. He expects operating profit margins to improve even further as Allegiant continues to figure out how to squeeze out more nickels and dimes out of its passengers.

Consider this
For 2013, the average ticket price increased by only 3.1% to $91.69, but the average "air-related charges" jumped by 13.4% to $40.52. This is an average increase of nearly five bucks apiece from every man, woman, and child who flew Allegiant.

Allegiant collected almost $288 million in fees (plus another $37 million in "third party fees") in 2013. It adds up fast! And this extra $40.52 per passenger is for many services not even charged by a number of the larger airlines. This means, all other things being equal, much of these extra fees fall to the bottom line fast.

Analysts also don't believe Allegiant is going to hit any wind resistance from this any time soon either. For 2014, they expect Allegiant to crack the $1 billion mark in sales by increasing 12.8% to $1.12 billion. For 2015, they expect sales to pop another 11.4% to $1.25 billion. That figure, if achieved, would be double the sales levels of just five years prior. For earnings, analysts expect Allegiant to grow 15.3% this year to $5.56 per share and another 17.6% to $6.54 for 2015. Much of that is of course from fees.

Allegiant may be catching the Spirit
Spirit Airlines (SAVE 3.57%) demonstrates even more success and growth rate for Allegiant to aspire to. While it took Allegiant hopes to double its sales over the five-year period beginning in 2010, Spirit Airlines has already doubled its sales back in 2012 and hasn't looked back.

Spirit Airlines hasn't reported its fourth-quarter and full-year results yet, but it is expected to show another 25.6% growth in revenue and a 65.7% increase in earnings per share. In the third quarter, revenue soared 33.4%, revenue per seat mile popped 8.9%, and adjusted net income exploded 130.3%.

In case you haven't guessed it, Spirit Airlines is known as king of the fee-niche industry and leader among the smaller airlines for successfully "getting away with" tacking on fees.

Spirit Airlines CEO Ben Baldanza mentioned that the airline is known for doing things differently than other airlines. It sounds like he's referring to the risks it takes by being the first to charge new or higher fees for certain things in the industry. Notable examples are $50 for a carry-on, $10 for a printed boarding pass, $3 for water, and even its customer service number is no longer toll free. If Spirit can figure out a way to charge you to use the restroom that'll probably be next.

Allegiant seems to be watching and slowly mimicking Spirit Airlines' creativity, as evidenced by its own revenue-per-mile increases.

Foolish final thoughts
The best part is that the customers seem to keep coming back for more, so maybe people tend to be more price-sensitive when they are planning a trip than they are about the day of travel. Allegiant has already reported preliminary January traffic data that continues the rapid growth trend. It showed double-digit-percentage gains in passengers and revenue per passenger, a double-double win. Look for Allegiant to continue to grow and impress, especially as passengers get accustomed to paying the extra fees for everything. Foolish investors looking to take advantage of this trend should take a peek at Allegiant as it "plays catch up" with Spirit Airlines.