I fail to see what's the gain between launching a rocket with 1 ton of preassembled componned or 1 ton of materia used by a space 3D printer to build those component.
Think along the lines of large objects that could not be launched from Earth pre-assembled, especially items that have large empty spaces inside of them.
Also. you can assemble some items in space that would collapse under normal Earth gravity. Building in space, means that the object only needs to be able to stay together in micro-gravity, which gives you a bit more freedom in structural designs.
The only way I can see a real gain is if most of the materia weight come directly from space. For instance, asteriod mining.
The current problem with mining materials in space is, we simply do not have the infrastructure in place to mine, purify and process minerals into finished metals in space. Sure it would be nice to see this some day, but in the mean time it's cheaper to launch your raw materials off Earth, especially if they're in the form of powdered metals, those pack very nicely into rockets. Besides I've heard there is some weird South African guy selling used discount rockets, not sure I'd trust him with my life, but with a pile of titanium powder, sure.
In short, you need to build the salt box pioneer shack in space before you build the steel furnace in space. We're barely past the salt box shack stage.