3D graphics. How much more expensive is rendering of 2 images 960x1080 than one 1920x1080?

Question

3D

Good afternoon.
This question arose for me because of the new helmets Oculus and Project Morpheus.
We have
a) 1 screen 1920x1080
b) 2 960x1080 screens with different streams
c) 2 960x1080 screens displaying one image from two angles (like eyes)
I assume that method a) will be the least expensive.
Thank you.