If you look under the "Materials" tab, you will see that the "sandstone" has twice the fixed startup cost of "strong&flexible", but its per-volume price is
about half that of "strong&flexible". ($3+$0.75/cm3 vs $1.5+$1.4/cm3) . On the other hand, "sandstone" parts need higher wall and wire thicknesses and cannot have heavy or extended overhangs due to the more complicated production process.
If you want your object to be textured, you will have to choose sandstone anyway (no price difference between multicolor and "plain" sandstone).
Either you mistook the first price quoted (probably for strong&flexible) for the sandstone one, or your model is not hollow anymore due to shapeways'
software attempting to fix a flaw in your model. (Or units got mixed up somehow, and your object is now 2.54 times bigger than planned - the model page will show you both material volume and dimensions)