3D print service Shapeways has reduced the cost of ceramic materials to "be more consistent with how we price the rest of our materials - by volume of material used instead of surface area."
It's a methodology switch, really. Instead of charging by surface area (which we suspect they began with as the ceramic process is all about the surface finish), they now charge by volume. They probably observed a relationship between volume and actual run cost after printing hundreds of ceramic items. Knowing that relationship, it must have been straightforward to do this switch.
It's a good process - start with something to allow people to get product, then refine it after you gain experience. It's the Shapeways approach.