Spells are treated like Skills. You are restricted to spells in the profession's Sphere and the Universal Sphere. Spell casting is under the skill category of Mystical Arts, so if the profession has that as a favored category, spells cost 2 DP per rank. Since any profession can pick up spells in the Universal Sphere, those professions without Mystical Arts, pick up Universal spells at 4 DP per rank.