Depending on the material, there is a base charge per file. Then they add a volume charge on top of that. With FUD, its usually cheaper to put everything in one file. Other materials, like brass, they won’t allow multiple objects in one file.
I upload in .stl whenever possible (I paid for a Sketchup plugin to do it) because the files are smaller. You can export .obj files directly from Sketchup, but they are larger. STL usually works fine, but sometimes it’s quirky and leaves out part of an object.