[Alneyan]

Notice how I write my answers. I basically managed to answer Jack by explaining that "both character level and the sum of your class levels matter to calculate the xp needed for next level".

It should have been, had I been less obtuse, "your character level determines the cost to reach the next level, and is the sum of your class levels". So a level 1 Warrior/level 9 Mage will have a character level of 10, and will have to spend as much xp to reach level 11 as a level tenth full-fledged Mage/Warrior/Butterfly.