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 Allies at Last's first ability). The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast that spell was.

Affinity for Allies reduces only the generic mana in Allies at Last's cost. The colored mana must still be paid.