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 (such as affinity for artifacts). The mana value of the spell remains unchanged, no matter what the total cost to cast it was.
Affinity for artifacts (This spell costs less to cast for each artifact you control.)
Flying (This creature can't be blocked except by creatures with flying or reach.)
less to cast for each artifact you control.)