How much it costs to fly a plane doesn't have much bearing on what it costs to get on board. (Spencer Platt/Getty Images)

Flying between Atlanta and Charlotte will only take you 226 miles but it'll still cost you: The average one-way flight is $286.

That works out to $1.27 a mile, the highest cost-per-mile in American air travel, according to the most recent Department of Transportation data on airfare. That's more than five times the median of the country's 1,000 most-flown routes, at 23 cents a mile.

Compare that to a flight between Las Vegas and Philadelphia, which costs almost $80 less and goes nearly 2,000 miles further. At less than a dime a mile, it's the best bang for your buck in the airline industry.

The gulf highlights how complicated and often baffling airline prices can be — and how market forces usually have more to do with airfare than the actual cost of flying a plane. The Department of Justice is investigating whether the country's four largest airlines colluded to manipulate those prices, asking whether they flew fewer flights in an effort to keep prices up.

This comes as the airline industry is raking in record profits as oil prices, which make up about a third of their expenses, have plunged and ticket prices have stayed put.

[Airlines are making record profits, but don’t expect a cheaper seat]

"Airline Economics 101 is that prices are set by origins and destinations, not by the cost structure, so cost plays a very small role in the actual setting of the price," said Joshua Schank, president of the Eno Center for Transportation, a Washington think tank. "That's why you get all those crazy pricing things in the airline industry where sometimes a shorter flight will cost you way more than a longer flight."

The five costliest flights per mile are among the shortest in the country, and they follow routes that usually aren't very competitive, like Portland to Seattle.

Only two major airlines provide service on four of the routes. Three airlines fly between Chicago and Cincinnati, which is known as one of the most expensive cities in the U.S. to fly to.

Only one of those five routes is serviced by a low-cost competitor to the legacy airlines. Southwest flies between Austin and Houston, but observers say the airline straddles the line between an inexpensive option and an established player, because it is now one of the largest airlines in the country.

"The prices start going through the roof, because they can get it," said Bradley Seitz, president of Topaz International, which audits airfare for corporations. "There's not as much competition in this market, or there's only two competitors in a market, and they'll start matching prices pretty easily."

Short flights also usually cost more in part because some costs are unavoidable, Vaughn Jennings, spokesman for industry trade group Airlines for America, said in an e-mail. Airlines pay rent to airports and fees to take off and land, for example, and those sorts of fixed expenses can drive up per-mile costs.

The five most expensive routes also fly to cities that are more business centers than tourist destinations. Cincinnati, for example, gained its reputation as a pricey airport because it tends to attract business travelers, the Wall Street Journal reported. They usually pay more because they often book later and have less flexible schedules.

"The money is always in the business travelers," Seitz said.

The five cheapest options belong mostly to the tourists, shuttling leisure travelers to the casinos of Atlantic City and Las Vegas and south to Florida.

It's a clientele that books months in advance and keeps a close eye on the price tag. Like any business, airlines try to get as much money as they can for every ticket they sell. With tourists, they've found that just isn't as much.

The routes also usually belong to the low-cost carriers.

Two of the three airlines flying between Las Vegas and Philadelphia, the cheapest route in the country, are discount carriers, and Seitz says that's forced the third, U.S. Airways, to drop its prices to keep its slice of the market. It's a phenomenon known as the "Southwest Effect," where competition from low-cost airlines leads to lower fares in a market.

On three of the cheapest routes, there are no legacy airlines at all. Low-cost airlines are the only options between Atlantic City and Tampa; Miami and Plattsburgh, N.Y.; and Las Vegas and Peoria, Ill.

Plus, long-haul flights usually have more competition because travelers can often find a cheaper option that connects through another airport, Jennings said.

"On longer-distance flights, there are lots of viable one-stop or even two-stop options that effectively create more supply in the market, and this can put downward pressure on pricing relative to some short-haul origin and destination markets," Jennings said.

Still, the Transportation Department data comes with a caveat: It doesn't include fees.

Fees, not airfare, have driven revenue growth in the industry in recent years, experts say. Bag fees alone accounted for $3.5 billion of U.S. airlines' revenue last year, a 29 percent increase over five years earlier, according to Transportation Department data.

And that doesn't include other fees, which are especially common on discount airlines that offer stripped-down service -- "the bare bones of get you from here to there," Schank said. Those airlines charge for some things that come free on traditional competitors, like space in an overhead bin.

If you're surprised at the price of an airline ticket these days and wondering what's behind the cost, here's what goes into the price of that airfare. (The Washington Post)