Yeah I ran some numbers though..

Base mana cost is 875 or so, reduced by 10% with a disc talent.

First one can be free with inner focus.

Paladin buffs the damage output on it with his aura..

Still not very efficient but it also spam heals the whole group at the same time. And with a prot pally tanking mobs, the speed of the fight can be slowed so that you only holy nova as he needs HP, allowing mana regen to kick in and stuff.

Plus, they'd be draenei, they get symbol of hope which is basically a mini manatide totem