I'm not sure how that would be cheaper
From what I gather, the ET's are built to be as light(flimsy) as possible so that they can mostly burn up on re-entry. To make them into habitable modules would require that they be totally renovated from the inside out....while in orbit. This would be kinda like building the shell for a station module on Earth, then have astronauts build all of it's internal components (life support, wiring and plumbing) in the dangerous environment of space. That just doesn't sound very practical to me.