Sometimes the ticket prices go up by a hundred bucks, or those awesome tickets you found were just sold out before you had the time to click Book. The most probable reason is that they're trying to make you act fast, before the prices rise even more. Read and get a simple way to save money on your Flight Tickets.
Flight tickets prices vary by location and search timesIf you were to book your flight from Brazil rather than the US, it would save a lot! Because flights are sometimes pretty cheap from Brazilian sites. For example, first use an American IP to search Kayak.com for flights from Los Angeles (LAX) to Melbourne (MEL), and I was quoted US$1842. Then, simply changed my IP by connecting to a Brazil VPN server, and did the exact same search, it saves around US$475.
Because you typically keep the same IP address for long periods of time and likely perform searches that personally identify yourself. When you search for the same flight for several times in a few days, the travel sites would increase the price of the same ticket.