Rates may depend on whether the instructor must travel to the student. Group lessons should cost considerably less depending on the size of the group. A good teacher has prepared lesson plans and a developed system. The main format of lessons should not consist of the student playing games against the teacher. When students have gained the experience they should record their games against other players and bring them to their teacher for analysis.
Video taping private lessons is a very effective tool.
In this way students can build a library of their lessons. Certainly the more lessons you take the more you will learn. Most of my students come once a week to a group lesson and can schedule additional private lessons according to their individual needs. For a serious student I would consider several hours of lessons each week as ideal. The best way to receive instruction is in person.