The cost for most spells 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 the same magicka cost as the built-in spell. However, there are a few notable exceptions, as noted in the tables. 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.
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)
Level 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
The base cost in gold that you must pay to purchase a spell is always three times the current magicka cost of the spell, taking into account your character's skill level and Luck. (The actual cost you pay is further increased based on how well you haggle with the merchant). Therefore, if you have any Fortify Skill or Fortify Luck spells/potions handy, you can get significantly better deals when purchasing spells.