The price of an airplane ticket.
Magnitude can depend on such factors as departure city, destination city, time of year, week, day of flight, special promotions, frequent flyer status, and alignment of Jupiter in the house of aquarius.
Airfares are the most unpredictable prices on the face of the planet. To give you a first-hand example of this, I had to fly to South Dakota this summer, and reserved my ticket online with Northwest Airlines. I accidentally booked two tickets instead of one (dust in the mouse, I guess). Before I cancelled the second order, I noticed that the first ticket cost just over $200, and the second ticket cost almost $2000!

I am not making this up, kids.

So let's take a look at why airplane tickets are so messed up, starting with something you and I have a lot of:

Class

To the average Joe, most airplanes have two classes, first class and economy class, and sometimes business class is thrown in for good measure. However, if you look closely at your ticket or frequent flyer miles statement, you'll notice that it doesn't have a class name printed on it: instead, there's a letter.

This is key in understanding how your airfare has been computed. To your eyes, the airplane has only two or three classes, but to Sabre or Apollo's eyes, it has a score of them, listed here in roughly descending order of price:

P - first class on three-class aircraft
F - first class on two-class domestic flights
J - first class on two-class international flights
A - discounted first class
Z - heavily discounted first class
G - first class upgrade

C - business class
Cn - nighttime business class
D - discounted business class
NC - business class upgrade

W - premium economy class
S - discounted premium economy class
Y - economy class
Yn - nighttime economy class
B H K L M O Q R T V - various levels of discounted economy class
Bn Kn Qn Vn - nighttime discounted economy class
N - student discount
E - shuttle service
U - frequent flyer reward flight
Wow.

"So what, exactly, is the difference between these ten classes of coach?" you're probably asking. Well, the reservation computers have quotas for each flight. For example, they might be instructed to sell ten "V" seats, then ten "T" seats, then twenty "Q" seats, and so on, each letter carrying a different price. In effect, this means that the fare increases incrementally up to boarding time, as the cheaper seat allocations are snatched up. Note that these letters bear absolutely no relation to the physical location of the seats: you can be sitting next to someone who has paid ten times as much as you have.

The most expensive levels of fares—P, F, J, C, and Y—are usually freely transferable between different airlines, which makes them useful if you need to change your travel plans on short notice. Cheaper fare classes are non-transferable, which means that you'll be SOL if there's a crisis developing in New York City and you're in Bangkok. Therefore, corporate travel usually uses the more expensive fare classes, while tourists buying their tickets from Travelocity or Expedia usually buy the cheaper fare classes.

When you actually get your ticket, it will have a fare basis code printed on it. A full-fare first class ticket with no restrictions would have a fare basis code of "F" or "P." An H-class ticket in low season, nonrefundable and requiring a seven-day advance purchase, would have a code of "HL7LNR." Unless you work for an airline, you don't actually have to know what these codes mean: on Travelocity and many other sites, they are translated into nice, caps locked English prose for your perusal.

In summary: If you want a cheap ticket, you have to find a flight where the cheap seats are still available, and make sure that your itinerary won't have any last-minute changes. Most travel websites do this automatically. One thing they might not do is go through all the possible connecting cities, so it is sometimes worthwhile to check fares going through different intermediate points: I got a ridiculously cheap ticket across the country once by making two connections in a Z pattern.

One thing you cannot do is "back-to-back ticketing." This usually involves buying two cheap round-trip tickets with a 7 or 14-day stay, and then using the first half of both tickets to travel on an itinerary that would require a shorter stay, thus saving money over what would be a very expensive ticket. This isn't technically illegal, but airlines find their own ways to screw you over if you get caught doing it: taking away your frequent flyer miles, for instance.

Standby

One thing you can do is buy a cheap ticket for a flight late at night, and then show up at the airport that day and ask to be put on an earlier, more expensive flight. Most airlines will happily let you do this, assuming there are seats open on the earlier flight.

This is, incidentally, how airline employees travel. On their own carriers, employees can fly standby at a very significant discount, only paying taxes and other fees on their itinerary. On other carriers, they can utilize a wide array of industry discount fares that offer 40-90% off the retail price, depending on the airline. See also nonrev and How to fly to Europe on stand-by.

International Fares

International airfares are more or less fixed by the International Air Transport Association, and the IATA's official prices are what you'll usually find on reservations networks. However, many wholesalers and other middlemen buy these tickets in bulk at reduced prices, and sell them to travel agents or local ethnic communities. So never take Travelocity's word on international airfares: call up your local agency and check out the newspapers at your local South American grocery/Irish pub/Asian market. The classifieds in big-city newspapers can also be useful in finding cheap tickets to destinations on the other side of the world.

Disclaimer: This applies to my home country, the United States of America, and probably Canada and Europe as well. It doesn't apply to some parts of the world, and if you would like to identify a particular regional exception you've come across, please /msg me and I'll node it up.

Sources:
http://www.iecc.com/
http://www.travelterminal.com/

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