When you create an object with Shapeways, the order price is determined
by the actual printed volume (not by the model's bounding volume). An
easy way to save some money (well, sometimes a LOT of money!) is by
making your object hollow. This short tutorial will explain the basics
of hollowing objects.
A nice thing that you can do in 3d studio max is use the measure option in the farthest right tab menue in the editing sidebar. this calculates volume in the current units that 3d studio max is set to. It also calculates the center of gravity/mass, which can help for the models that need to be able to stand(also can use reactor to do physics calculation for it).