A home buyer might fill out a mortgage application online, where some banks charge lower fees. That seems fair because it saves labor costs for the bank.

In Chicago, you can make a reservation at Topolobampo, where the serrano-and-garlic-marinated Maple Creek Farm pork loin costs $31. To eat at Frontera Grill, a less expensive restaurant served by the same kitchen, you have to wait at the bar until a table opens.

In each case, there is an obvious tradeoff between what you pay and how you get it. The same can even be true with soda: people will spend $2 to buy a cold Coke at a convenience store when a warm one, bought in bulk, might cost 50 cents. But a Coke that comes from the same vending machine feels like the same product, whether the weather is stiflingly hot or comfortably breezy.

"Where companies get in trouble is where they base it purely on supply and demand," said Mike Marn, director of pricing services at McKinsey & Company, the consulting firm. "Those are the situations where consumers really get up in arms. If you're going to take advantage of the demand, you have to be able to say with a straight face that there's a benefit that goes with it."

Sometimes companies add the benefit later as if it, rather than level of demand, is the reason for the higher price. Airlines give bulkhead seats to fliers who buy expensive, last-minute tickets. Broadway Inner Circle, which sells marked-up last-minute tickets to hot musicals and plays, organizes dinners at which its customers can eat with a producer or actor after a show.

If anything, people seem more accepting of price changes on expensive items, where such frills are easier to add. On a can of soda - or a copy of Newsweek, which charged as much for the issue after the pope's death as last week's issue featuring dinosaurs ("Beyond T Rex: How They Really Lived") - even a few extra cents can seem like an injustice.

Part of the issue may be simple recognition: Everybody has an intuitive sense of what a soda or a magazine should cost, but determining the value of an airline ticket is trickier. On a costlier item, there is also more room for the sort of benefit that makes the markup feel legitimate.