Silicon is really expensive. Would be a lot easier to just bake different sizes of round cakes, assemble them and simply bevel / trim them into a sphere.
That might be what I need at first with 12" diameter sphere cakes. However with larger sizes I would definitely need sphere sections of silicone molds. I mean I am actually envisioning up to 5 feet diameter spherical cakes and no way can that be baked whole. In fact that could take up to 20 different sections baked separately and assembled later.