last edited by DBatterskull
@John-Cox It depends on which Sphere. For Sphere of Resistance, both both the Sphere and Hollow One's effect apply in the phase of casting a spell where you figure out and pay alternate/additional costs (Rule 601.2b and Rule 601.2f). So for example, if you discarded/cycled one card and there's one Sphere, it would cost 4. If you discarded three cards and there are three Spheres it would cost 2, and so on.
Trinisphere is a little different. It checks after additional/alternate costs and always makes sure you're paying 3 (Rule 601.2f again). So no matter how many cards you've discarded/cycled, Hollow One always costs 3 in the face of a Trinisphere. Hope that helps!
Source - Being a L2 Judge