Check your STL with netfabb studio basic to see how many polygons it actually has - perhaps your software uses
different settings for on screen display and stl export, and the STL mesh is much coarser than you intended.
(That is assuming the shapeways preview actually resembles your model - an alternative explanation would be
that most of your object was thrown away during import - if this happened to internal geometry of a supposedly
hollow object, it will drive the price up)