Seems like a lot to me as well. However, first place to look is your lease agreement. What does it say--what did you sign up for?
Most states do not regulate late fees--what you sign for is what you get.
Call a GA attorney and have a quick chat (a GA attorney will be familar with local/state ordinances or laws that may apply in your case) --many offer free consultations and most will be happy to give you a quick answer to your question.
READ THIS BEFORE CALLING OR EMAILING ME: I am licensed in the Commonwealth of Virginia, addressing your issue does not create an attorney-client relationship and I AM NOT providing you legal advice. You should speak with an attorney to whom you have provided all the facts, before you take steps that may impact your legal rights. I am not obligated to answer subsequent emails or phone calls unless you have hired me. I wish you the best of luck with your situation.
A late fee of $100 on a rent of $700 is not illegal. The real question is whether or not this amount is usury. The answer to that depends on what your lease states.
Most leases assign a fee whenever rent has not been paid by a date certain. This fee is legally classified as "rent" rather than a "late fee". As such, it is not controlled by the percentage rules for interest. Further, if this fee is in the lease, and you signed the lease, you have agreed to it. Once you agree to something, you are obligated by your agreement unless you can show the "thing" you agreed to is grossly unfair or you were coerced into making the agreement.
The above answer is a general explanation of legal rights and procedures. It does not constitute legal advice. Nor does it establish an attorney-client relationship between the individual posting the question and the attorney providing the answer.