Shapeways Price Estimator
Shapeways computes (most of) their prices based purely upon the volume of material that is consumed during printing.

Enter the outer dimensions of your model:
 Wall thickness (0=Filled)
 Inches (default is millimeters)
Total Volume   cm3    
Startup $ Volume $   Price Material   Note
1.50 1.40    WSF    
3.00 0.75    FCS    
2.50 2.99    WD    
2.50 2.99    TD    
2.50 2.99    BD    
5.00 2.39    FD    
5.00 3.49    FUD    

* 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.