Some advance research can help you know if you want to travel with a carry-on bag only (to save a baggage fee) or to bring your own lunch rather than buy one at the airport or on the plane. Some websites, including offer on baggage fees at major airlines.
Your calculations can even extend to costs after your flight lands. Can you save by using public transit to get to a hotel or to a rental-car agency further from the airport, where rates are lower? If you need lodging, websites like Quikbook or can help you shop around.
