Elemental Expert is a star within the Constellation The Apprentice in Elder Scrolls Online.
With each point invested, the increase in the Flame, Frost and Shock Damage diminishes. That is to say, for the first 10 CP invested, you would gain a 2.9% increase in Damage. However, the next 10 CP spent (making a total of 20 CP spent), will earn you only 2.56% more Damage. This is 0.34% less than the increase gained in the first 10 CP.
As such, spending just 37 CP into Elemental Expert will already gain you a 9.09% increase in Flame, Frost and Shock Damage. This means you will have gained 50% of the Maximum increase possible (which is 15.0%) by spending just 37 out of the total of 100 CP. Therefore, beyond this point, the increase Damage received per 1 CP used is very small and not as worthwhile.