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 that of Argivian Phalanx). The mana value of the spell is determined by only its mana cost, no matter what the total cost to cast that spell was.

Argivian Phalanx's first ability can't reduce the {W} in its cost.