Affinity for artifacts means “This spell costs less to cast for each artifact you control.”

To determine the total cost of a spell, start with the mana cost or alternative cost you’re paying, add any cost increases, then apply any cost reductions. The mana value of the spell remains unchanged, no matter what the total cost to cast it was.

If a spell has multiple instances of affinity, each one applies. For example, if you somehow control two Sami, Wildcat Captains and you control two artifacts, each spell you cast will cost less to cast.