That varies widely. Some teachers don't charge and teach in their home or a public facility. Others charge a great deal and have to deal with overhead such as dojo rental, phones, advertising, etc. Most instructors have other businesses that provide them income and do the martial art for the love of the activity.
On a good day of teaching, I can make $50 - $60 dollars, my best was $132. On other days I make $8.
Not much. A lot of martial arts instructors teach in addition to having a full time job. It isn't a career to make money in.
Most martial arts schools only teach a few hours a day, if that. The places I know only have 20 to 25 hours of classes a week. Even if you get in a full sized class, that can be limiting.
The average cost per month varies greatly, from around a low of $75 to as much as $165. 100 Students at $165 a person a month is $16,500 per month over a years time that's $198,000. Take out your costs for the dojo, utilities, liability insurance, advertising, equipment costs, insurance for staff (if you have any staff to pay) and taxes. Being the owner is the benefit, an instructor paid by the owner will struggle with about $25,000 per year (and that is on the higher end scale).
That's if you can get 100 students! And you sure aren't going to do that day one. It will take 20 to 30 students just to cover the overhead.