Shapeways computes (most of) their prices based purely upon the volume of material that is consumed during printing.
* This estimator is based upon a flat rectangular (hollow) box with the dimensions you select above.
It will give you a rough idea of how much a model will cost. If your model is highly detailed, it's often the same as making the walls slightly thicker. If your model contains a lot of holes, then the price obviously would be less.