I just rerolled priest today and found out that all priest spells cost way to much mana.
examples
Heal-233 mana
Flash heal-131 mana
Power word: shield-167 mana
Smite-109 mana
Shadow word: pain-160 mana
I am sure you will look into this it basically every spell. I did all 3 of the world boss and while doing so i found myself puting up a dot and wanding trying to keep my mana up.
Thanks for reading my post!