About your other questions, I really don't think that a formal qualification is that important. Unless you're thinking about public schools. In the private ones, I believe that how good you can be will be the think they will search for.
Going to an English school will certainly be the most popular choice for students around here.
The fact that you're a native English speaker would give you a great advantage in the private sector for sure. You could easily work in a school and by yourself.
I guess if you can found a niche you can earn more, like this guy
I think he teachs online and also in some school.
I also think a formal qualification is not mandatory but can helps to find a formal job. If you decide to focus on some exam ou business maybe the formal qualification don't matter.
As you must know, our currency is Real (RS) so I am going to give some examples in Real.
A privete tutor cust about R$ 50,00 per hour in my city. (I live near São Paulo city and there are 600,00 people in my city).
I payed about R$ 250,00 in a school with 2 classes per week during about 45 mimutes each class. There are schools in my city that cost R$ 150,00 per month.
If you have some doubt, I can talk to you about it.
I don't have children and I 'have no ideia about costs I also live far from SP or Ribeirao Preto.
I tried to found out on google but I think they don't like to show up this information.
I found a news talking about a ranking of the best schools in Ribeirao Preto, I guess you can talk straight to them.
I don't have any idea, because I live far from Ribeirao Preto.
I believe that it is better you to write a new question focusing on Ribeirão. Brazilians like to help people and I am sure that someone will help you!