I don't understand your post above, Pollo.
What are you talking about there?
A Child of Illian has swarm attack, size 1, and a CC weapon which gives it Dam 6.
A Karnophage has swarm attack, size 2 and a natural attack of Dam 10.
That means it takes 3 Children of Illian to swarm a trooper size 2, and the 4th+ gets +4 to CC and Dam.
It takes 2 Karnophages to swarm a trooper size 2, and the 3rd+ gets +4 to CC and Dam.
The only major difference is that a Karnophage can not hurt something with Inpenetrability, but the Karnophage does more base damage against things that don't have Inpenetrability.
So the cost on swarm attack doesn't change, but a cost on Inpenetrability is levied against the posseser of that special.
Understand? It isn't that swarm attack should be cheaper for some units, it is having Inpenetrability (and a very limited defense it is) is a cost on the model that has it.