Most airline tickets are dynamically priced. This means that the price changes depending on when you purchase any given ticket. This is often a function of supply/demand (which often follow holiday schedules) as well as certain promotions.
Variations can be as short as minutes, though prices normally change day to day or week to week. Prices often increase closer to the date of the journey, but in some cases they can also drop.
To combat resentment and encourage impulse buying, many airlines have a return policy on the differential if the prices of the ticket drops after you purchase it. But you have to file a claim with the airline proving that has occurred. You have to know that the price has changed and searching for your exact ticket every day probably isn’t the best use of your time.
Yapta is a website that automates this search and notifies you when the price of at ticket drops, so that you can file a claim to refund the differential.
Here is an example of a flight with quite a variation in price.
I hypothetically purchased this flight for $900 last month. Yapta kept checking the current prices and sent me an email with a price change alert. $227 is quite a savings on $900.
To setup an alert, search for the flight you purchased (or is thinking about purchasing) and enter the price you want to set as the baseline. You can then set the alert criteria – for example, how much of a differential before an alert is sent.
Try out Yapta and write your thoughts in the comments!