They forget to state how much or how little they will pay the teacher
Well, a bit of mathematics is always useful in these situations.
They charge a minimum of 295zł for 16 lessons in a 6 person group. So you can count like this :
6 people in the group - they earn 1770zł per month. (110zł per lesson)
5 people in the group - they earn 1475zł per month (92zł per lesson)
In a 4 person group, they charge -
4 people in the group - 1280zł per month (80zł per lesson)
3 people in the group - 960zł per month (60zł per lesson).
Their operating costs are minimal because they use Skype and similar, so anyone that wants to work for them can easily negotiate based on the numbers above.