The magicka cost for leveled spells rarely matches the value you would calculate from the Spell Cost equation. In other words, if you were to create the same spell at the Spellmaking Altar, it would have a different magicka cost than the leveled spell. Spells that are a bad deal, i.e. more expensive than a custom spell, are in red; spells that are a good deal are in green. The magicka cost if you were to create a custom spell at the Spellmaking Altar is shown in parentheses.
Magicka cost for spells is dependent upon your skill in the school (as modified by Luck, see Magic Overview). The provided baseline values are the magicka cost when your skill level is 33. For other skill levels the magicka cost can be determined from:
Cost * (1.4 - 0.012 * Skill)
Difficulty indicates what skill level you must reach (or surpass) in order to use a spell. Skill Level 0-24 = Novice, 25-49 = Apprentice, 50-74 = Journeyman, 75-99 = Expert, 100 = Master.