Commonality is just a part of maintenance costs. The other part is directly linked to the airframe. Current GW3, I've got 93 A320 and 29 IL96, for respectively 5.5M$ and 4.5M$ of commonality costs(plus trinkets for the engines, though the Aviadvigatel engines are costly on this side). Per month.
Still, my monthly maintenance costs are around 20M$, because each individual plane has its own costs. That's normal. It's an addition.