This is actually a very mana efficient spell. It is a PBAE spell, so the more mobs you can get close to to cast it, the more efficient it gets.
If you are in a AE group and the puller bring 10 mobs, you do 10500 damage for 780 mana. And if you have a mana preservation item and Spell Casting Mastery, the mana cost for the spell is less than 700. If you have an Enchanced Damage focus, you can do even more damage (20% more) plus you have a chance to crit as well. (With crit and ED4, and MP4, its possible to do 23000 damage on 10 mobs for < 700 mana.)