Pricing per Theme

Hi ,As people studying in Macat usually are committed to their professional life  like my self or they are still Studying . I believe that it is better to make ur pricing per book , or course or Theme not per time . As it's not easy to allocate specific time for macat and stick to it. We can not project how will the month go at work. I want to study one theme . everytime I think of paying for a month i say may be i will have work pressure and won't be able o give it more than few hours. and i can't afford paying for a long period.