Do you charge by weight?

Most practices charge a third to a half less for porcelain onlays than full crowns. yet I've never heard a valid reason why this is so.Porcelain onlays, are a perfectly reasonable treatment option for broken back teeth. They have proven strength and longevity as well as the decided advantage of being far more conservative of existing tooth structure.Porcelain onlays are more technically challenging to do well, take more time to cement and cost roughly the same from the laboratory. So, why do they cost less?There is no reason that I can come up with. Unless dentists charge by weight.