Preparing a model for 3D printing, that was not built to be printed, is not a simple task. With some exceptions, the entire model needs to be a continuous mesh. Further, there needs to be an inside and an outside.
I know Shapeways.com charges per cubic centimeter of material used, so non-hollow shapes cost way more than hollowed shapes. They also have a tutorial section at
http://www.shapeways.com/tutorials
The tool chain in AC3D for 3d printing is pretty much in its infancy.
-There is a new plug-in for 7.2.17 that detects edges that do not have exactly two surfaces attached. It does not detect other problems like flipped normals and intersecting planes.
-The extrude by normal tool doesn't always generate parallel surfaces so its hard to make an "inside" surface in some cases.