For the bonus that a caster receives for learning multiple spells of the same type, see Spell Affinity
Spellcasting, Skill use, Gem charging and Imbuing (but not stronghold planting/revoking, or Arcane Marksman) are done at a 25% discount in MP cost. Cost of "Charged Attacks" added to spells are not affected if the cost of the addition is less than 4 MP (e.g., Arcane seeker and Eldritch Blast are not discounted, even if used together, since each costs less than 4 MP).
This MP reduction stacks with the discount granted by the Darksoul Cabal.
Arcane Affinity does not reduce the cost of Mystic Vigor.