Charges or prices emblematically go in two extreme directions at last minute travel last minute travel - high and low. Charges skyrocket when the demand is higher than the number of seats on the flight. This can be due to last-minute business travelers, holiday season, and weather disruption, etc.
The prices can plummet down when there are lots of seats left at the very last minute. I have personally seen this when you travel odd hours or at different times of the year, maybe odd!
It can happen during the holiday season when airlines anticipate enormous demand, but demand is less. In the end, they reduce prices to salvage remaining seats. You have to understand how ticket prices are computed nowadays. Last-minute fliers are generally more likely to be business travelers, or they have emergencies that they are willing to pay more.
Airlines take advantage of this and price the ticket much higher. I would say for domestic flights; you should try to purchase two weeks two months ahead of time and international flights two to six months ahead. If you do not wish to monitor the flight prices yourself, you can sign up for a flight deal alert service such as Flight Deals Hound.
It will alert you automatically when a cheap flight near your location is found. Takes on all the work for you. Overall, airlines do an excellent good job but continuously monitoring the demand and supply, and then adjusting prices accordingly.