when i taught at a recreation center, it was not much money but a small stream of income to my main job, so it you teach at a few places (i taught at 3: a rec center, karate school, and a wellness center around 1996-1998) , you have some additional supplementary income from a few places to your main day job income. There really is not a lot of money in teaching unless you have your own school with good marketing.
good marketing involves having a good background/experience:
1. champion in big tournaments (and having awards, medals, trophies),
2. your lineage and recognized by various masters/certifications/apprenticeship-disciple, teachers who can vouch for you.
3. judge/coaching experience, - ability to produce champions. judge in tournaments.
4. interaction with other coaches/sifus/judges in tournaments, and martial arts community.
5. having a good after-school program for kids, ability to teach kids and adults.
6. website, blog, youtubes with real skill displays, etc.