Level: Very high levels and very low levels of English often require a lot of prep and many teachers charge more because of this.
Student Age: Again, the money is in the opposite extreme. Very young kids and business people often pay more for private English classes.
Type of class: If it's general English you're going to be charging less than teachers who do exam prep or EAP for example.
How often you meet per week: Some teachers give students discounts for multiple classes per week.
How long the classes are: A teacher may charge 50 for an hour class and 90 for a two hour class.
Commuting time: I'm against charging more for students who live further away. You wouldn't ask your boss to pay you more because you have to travel further than your co-worker. If you don't want to travel too far simply don't accept the student.
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