The cheapest way is to pass in the minimum hours required. To do so you need good instructors. Doing it on a continuous course will be better than having long spells between lessons.
The difficulty of the written exams will depend greatly on your recent experience of exams.
A school where, if your instructor finds you have a problem he can't fix, he can pass you to another instructor, is an advantage.
(In 1964, my Instructor stopped a circuits lesson, to avoid wasting my time, and another Instructor sorted my landings later that morning.)