How much does it cost to upgrade to business class?
When you want to upgrade a flight you need to pay the difference between the original fare purchased, and the new business class fare that is available at the time of upgrading. As fares vary according to availability, if the business class cabin is full, that difference in fare can be expensive.
If the flight you are intending to upgrade was ticketed in a package with other flights, you will need to upgrade the whole package to business class, and that might also increase the cost of upgrading your flights.
There are some airlines that enable promotional upgrades at the airport if there is availability in the business class cabin on the day of departure, so it can also be worth checking directly with the airline.