TBE is a buffer! Salt water is not. During electrophoresis you will produce H+ and OH- ions, ie the ph will change. This is why you need a buffer. Ph changes are not good for biomolecules. Using salt will produce chlorine, a pretty strong oxidizer and not good to breathe. Oxidixers are not good if your goal is keeping protiens and nucleic acids intact. Also TBE is used for nucleic acids as the boric acid forms a complex with them, improving the resolution. No ideas off the top of my head for a cheap substitute.