Shapeways assumes solid material usage last I checked, so make the part thin walled and compare.
I love their precious metal prints (which many are 3dp wax investment castings in truth)
I had my wife and I's wedding rings printed there for much less than anywhere I could buy them, and they were custom.
I've gotten a handful of plastic parts over the years with their higher resolution offerings. Compared to the cost of the high end machines the prints are well priced.