Not quite. There's public liability and professional indemnity insurance, too. You mentioned insurance, but I'd be surprised if what the students pay covers it.
If someone breaks their leg and sues the instructor, it could cost them literally hundreds of thousands. So they get insurance. How much it costs, well like all insurance it varies a
lot, but basically the cheaper the insurance the less it covers.
It's the same with personal trainers. There the premiums are typically not very high because it's generally low-risk stuff. But if your sport involves belting people in the noggin then it'd be expensive.
There are also some kickbacks in most martial arts. If your school is part of a larger school, they have to pass some money up the line. Goju Kensha Carlton would have to pass money to Goju Kensha Victoria, then to Australia and of course to Japan. Apart from the immense egos involved, this is one of the reasons some instructors invent their own school, Goju Kindred or whatever.
Providing teaching also counts as a "service" and so is subject to the GST. This means the owner has to keep receipts of everything and do a Business Activity Statement every quarter. In practice this means a couple of hours a week with paperwork, and a lot of money to accountants.
The only way to become rich in martial arts is to get into movies