At Future.Travel, we deal with this sort of change of plans every day.

Generally speaking the charge to change a flight that someone missed is made up of various components. First would be the no-show fee. This is a charge for you when you miss a flight either by error, accident, or on purpose. That fee is up to the Airline. It is normally disclosed in the fare rules by the airline.

Then there is a change fee that is added to that. This is the cost of changing a ticket. It is also set by the airlines. It is also found in the fare rules.

Then you must add the change of fare basis. This is the difference between the price that you paid and the new price available for the flight that you were intending to take. And in some cases this might be zero, and in other cases it can be thousands of dollars. Again this is a rule set by the airline. This one is a bit more tricky as it is not a straightforward value, and changes depending on the flights called. Everything is set by the airline.

Finally the agent, or an OTA like Future.Travel, that you are using to buy the ticket, or even the airline, will have an administration fee for the service charge of changing and going through the process. On average this sort of change contact between one and four hours to complete.