normally you would pay an admin fee and the difference between what you have paid for the ticket & the cost of the ticket now. who is the flight with ?
i did a name change last year with jet2, there was an admin fee to pay and I didn't think there was any difference in price as on the internet the price of the flights were still the same......
but...... with Jet2 you can't do a name change online, you have to ring up, secondly the price of the flights online is cheaper than via the call centre, so had to pay about £50......... they have you all ends up