How much does your overall mark drop if you fail an exam?

That depends on how many other things contribute to your overall course grade. If your entire grade is based on 4 exams, for instance, then failing one would affect 25% of your grade. If the midterm is worth less because of papers, homework, attendance, and other assignments, then it could affect only 10% or less of your grade. It also depends on how badly you fail. Failing at 50% is much better than failing at 0% (not taking it, or not knowing anything on it). At least with 50% you got some of the points, and so it makes it easier to do better later in the class and raise your grade.