The XP cost given for an Extraordinary Ability is the initial cost to purchase. Depending on the ability and the method with which the character learns it, the GM may decide that it takes a certain amount of time to manifest, or no time at all! Characters that have an Extraordinary Ability listed as a Racial Ability for their species automatically gain that Special Ability at character creation at no XP cost.
Anyone can spend XP on improving an Aptitude associated with an Extraordinary Ability, which uses the same XP costs as any other Aptitude. Only Shapers can improve the other aspects of an Extraordinary Ability by spending the required XP on its Effects. Each Effect has a different XP cost associated with it, and 1 or more levels. To improve an Effect you must spend XP equal to the cost listed, multiplied by the new level. Where there is a limit to the number of times an Effect can be improved, this will be listed as the Max.
As an example, a character that has learned to cast that fine staple of the fantasy genre – the fireball, has several options open when spending XP to improve it. He or she may choose to spend XP on improving their Aptitude, thus making it easier to cast, or they may choose to improve one of its Effects to increase its range, area or potency (the damage it causes!). It’s not an all or nothing situation and they don’t have to increase everything at the same time. They can pick and choose what they spend their XP on improving.