headliner drooping b\rear window

Hello, you cannot really repair a headliner. Once it is sagging or damaged, it's not something that can be reformed and stabilized. You are correct, replacement headliners are very expensive, even though they do not typically contain the air bags themselves, it is labor intensive, and sometimes requires removal of a windshield or back window to get into the vehicle, adding more needed parts and labor.

Visit auto upholstery shop - some of those folks can work wonders!