So, when a bag is lost you may assume the airline will cover the cost to replace it.

But 7NEWS reporter Dayle Cedars found that's not always the case.

"It makes me really bummed that I chose to fly with Frontier," Colorado resident Carrie Caldwell told Cedars.

About two months after Caldwell flew home from Dallas, Frontier Airlines says her bag is lost forever, and it's time to settle.

But Caldwell says the airline will only pay a fraction of what her clothing and luggage cost her, because Frontier is factoring in the depreciated value of the property, based on wear and tear over time.

7NEWS checked with the Department of Transportation. It says every lost bag can be reimbursed for up to $3,300.

Caldwell says her stuff is only worth $1,200. But Frontier says it will only pay $700.

For example, Caldwell bought her suitcase in 2007 for $150. Frontier will only give her $74 for it now.

Her jeans originally cost about $50. The airline will pay about $25.

"How do they expect you to replace them?" Cedars asked.

"They don't," Caldwell replied. "They say since you have had it for so many years, it's not worth anything anymore."

"But I still have to go buy a new pair of jeans," she added.

Chad Gruhl, a hospitality professor at Metropolitan State College of Denver, said depreciating the value of a passenger's lost items is not only unfair, it's a bad business practice.

"There are very few people that are going to try to take advantage of a situation, and this is good example," he said.

The airline "lost the luggage. Pay for it," Gruhl said. "The passenger didn't ask for the luggage to be lost."

Frontier Airlines refused to talk to 7NEWS on camera about the issue.

7NEWS checked and found that Southwest Airlines also reimburses for lost items at cost, minus depreciation.

American Airlines and United say they have the right to depreciate the value of luggage the airlines lose. But experts 7NEWS spoke to say the major airlines won't.

"Will you ever fly Frontier again?" Cedars asked.

"No, never, ever again," Caldwell said.

And for that reason alone, Gruhl said this was a bad business decision – that travelers will be talking about.