Teacher resume/cover letter question?

I teach in a small private school and love my job, but am looking to change schools because I live 3 counties south of where I work, and my commute takes me an hour to an hour and a half most days. That's not even so bad, but the gas is killing me. As I said, I work in a private school, so I only make about 22k a year, and that's a huge issue, too. Anyway, I need some ideas about my cover letter. How do I word my reason for searching for a new job? Do I state that as the reason? Do I mention it at all? I just don't really know where to start. Thanks in advance!