The 10News I-Team took a closer look at the low fare and found that when flying with Spirit Airlines, passengers get what they pay for -- literally."I think they're trying to trick people," said Michael Belch, professor of marketing at San Diego State University. "There's got to be some sticker shock when you go online and you say it's $9, and then by the time you get off and you've made your reservation, it's $110 or whatever it is."A $9 fare online quickly went up once government taxes and other mandatory and optional fees were added.On its website, Spirit Airlines calls it "offering ultra low base fares with a range of optional services for a fee."However, others call it "nickel-and-diming."Spirit Airlines became the first airline to charge customers for carry-on bags in 2010. A personal bag that fits under the seat is free, but a carry-on bag that doesn't fit costs between $20 and $40. Checked bags are extra, as are advance seat assignments.Additionally, once on a plane, a drink of water or a soda will cost a passenger $3.A Spirit Airlines representative told 10News, "All fare information and options are clearly displayed on Spirit.com, allowing customers the freedom to choose only the extras they value. Customers have full control to review and make modifications to their selections before finalizing their transaction."Some passengers told 10News they liked the options."I love their prices," said Maria Castellano, a Spirit Airlines flier who said she's not bothered by the company's practices. "As long as you read everything there won't be any surprises.""They want to charge you whatever they can," said passenger Robert Crisostomo. "Just be conscious of that, otherwise you will pay those extra fees."One fee that's not so easy to get rid of is the "passenger usage fee." Despite its official-sounding name, it is not a government or airport-imposed fee, but something Spirit Airlines invented. Passengers are not charged the $16.99 per one-way travel fee if they purchase their tickets at the airport.10News went online and found flights from San Diego to Las Vegas over Christmas weekend advertised for $9 each way. Without any extras on Spirit, a roundtrip ticket for one adult came to $73.38. It was $18 for the flights, plus $55.38 for taxes and fees. That price was lower than flights 10News found on the same days with Southwest Airlines, which was about $119 roundtrip.However, passengers on Spirit pay for bags and if a carry-on was added each way, Southwest beats Spirit's price, which ends up being about $133 -- a lot more than the advertised $9 fare.That's why Belch protests the company's promotion."You actually start to push -- in my opinion -- the bounds of deception," said Belch.Spirit Airlines may soon have to stop advertising those famous $9 fares. Starting Jan. 24, 2012, airlines will have to disclose all government taxes and fees in their advertised prices as part of the airline consumer protection rule issued by the U.S. Department of Transportation.