As Rob says, it depends on the individual centre's policy on repeated levels. It seems that you're getting a fair deal on jump costs at the moment. Some centres charge reeats on a "slot" basis, where you pay only for jump tickets for you & instructors, kit hire & packing, but that can easily come to $80 per jump anyway. Plus, these centres would charge more on your early repeats where you had 2 instructors!!
"The centre's costs for the repeat jump are the same".
Not necessarily... Assuming the centre is giving decent training for each level, the centre saves at least 3 hours of instructor time for a repeat Level 1, and a good 1-2 hours for each successive level. AFF is not intensive in terms of the number of times an instructor jumps with the student (a perfect 8 level AFF course is only 19 slots). It IS intensive in terms of the time the instructor spends training, briefing & debriefing the student, but much of the training for a repeat level has already been done, so "slots + kit hire + say... $20" is the "no profit" option taken by some AFF Schools. So... $100 or so is fair for L1-3, $80 for L4-8.
Of course, the person who completes AFF in the minimum jumps tends to be in the minority, so some schools cut back on the base price & charge full rate on repeats. ALso, with larger schools, much depends on how the school pays the instructor. If they pay the instructor by the hour, they can easily lose money with a slow instructor or pupil. If they pay the instructor a set rate per jump, they risk the instructor giving "skimpy" coaching in order to get the student in the air & thus earn money.
Bear in mind that unless the AFF instructor is a close personal friend, they are not instructing for fun, but for money. This is a business so it has to provide a living! That said, while the school or instructor has to show a profit, they also have to give "value for money" or risk losing custom. Not easy!!!