I reckon that you can charge up to $40 if it has that many exams in it and that the questions are very original yet relevant to the study design. You might want to have the first few exams set as "basic" and then move towards gradually to "harder" exams which contain more VCAA-level difficulty questions. For methods this year I did over 50 exams sets of exams (some repeated) but I personally HATED kilbaha cos they were relevant to VCAA. To be honest, most people who get to the stage where they are near the exam just want to complete exams that will be like the exam they are going to take and anything harder will make them feel demotivated and unwilling to perform their best.
Also, for the chapter tests/topic tests, I would also include a few MC questions and seperate calc and non-calc questions since some of my sacs this year were not all calculator and some had MCQs in them.