Caster hybrids that have a spec that does not or to a lower extent uses mana will get the passive ability to increase their manapool by 400% for specs that do use it.
Why those hybrids have such high base mana costs is because their base mana is still the 1/5 value of that increased manapool.
In order to compensate and make heals equally expensive they artificially scale the base mana cost to reflect the cost of pure mana users.
So 4% of 100k is still the same as 25% of 20k.
There's really nothing hard about it when you come to think about it.
How big the manapool will be doesn't matter. Percentages are always relative so they will even out obviously.