keep in mind this isnt factoring in fatigue. the level of the spell is the power cost. most of the spells have strategic spell level requirements to maintain specific builds.
in most cases spells start at 100 fatigue and go to a minimum of 10 (bolts and heals have a min of 50 fatigue) -- a reduction of 10 per level above the spell.
manastrike 3 for example:
level 6 = 100 fatigue
7 = 90
8 = 80
9 = 70
10 = 60
11 = 50
12 = 40
13 = 30
14 = 20
15 = 10
which means that manastrike 2 is much better until you hit 12-13 or so... this is all a bit off topic though, plz ignore
