the mana cost is per total spell cast, not per target hit by those spells. AoE spells cast normal since they would already hit all the targets, single target spells cost double and hit every target.
If you're only casting 917 once (1 cast of 917 for every creature in the room, up to your target limit) the total cost is only 34 mana, so you are charged the minimum of 50.
You could cast
512 - aoe so base cost = 12
512 - aoe so case cost = 12
917 - single target x2 = 34
903 - single target x2 = 6
903 - single target x2 = 6
903 - single target x2 = 6
12 + 12 + 34 + 6 + 6 +6 = 76 mana which would still only cost you 50 mana because of the water lore and every creature up to your target limit would be (assuming they fail all wardings) encased in ice, earthen furied, and hit with 3 minor waters, in 1 spell cast for 50 mana.
Core tap is good.
Mithrilschlong, 2015-03-10 to slightly later on 2015-03-10. You will not be forgotten!
usable Meteor Swarm, late 2020-12-30 to early 2020-12-31. You will also not be forgotten!