If you order through your 3D prints through Ponoko or Shapeways there are several different materials and finishes that can be used to produce your final product. There is an option to select the material (Shapeways and Ponoko) and the cost is then recalculated accordingly.
The preloaded models have been hollowed out to save on printing costs. See link to hollowing out models.
If you are printing at home then you will have some experience with your low cost 3D printer and the various materials. Please do share your experiences and prints with these files to contribute to the discussion.