Do you think a martial arts teacher earns much money?

If not they could always start mugging

Doubtful, by the time you factor in insurances required, venues for classes, his/her own classes and gradings, plus taxes and healthcare, then how they get by should they be unable to work, i'd say the left over will be peanuts.

Most do not

Probably as much as any other gym rat who monitors a gym.

Depends how many classes they teach, students they have.

I don't think they do