Multiple camera in iphone project expensive?

Howdy all, and thanks for your attention.

I am doing a top down camera (non-orthographic to get some parallax scroll) 2d game and am using multiple cameras as children of a parent cam to render different objects in layers in the scenes to stop the object from tearing and sorting through each other. Is this an expensive thing to do?

Thanks,

-Goody!

I've got several cameras in my iphone game as well as a load of physics and particle effects and it runs fine, go nuts.