There is no such thing as a renewal fee.
You can keep using the student version of the release you purchased as long as you are a student and satisfy the other requirements. When you graduate, and then become a student again, you will again be allowed to use that same release.
If you want to use newer releases, you will need the subscription maintenance service. That carries a yearly fee (although you might be able to pay for two years in one go). I believe the amount Mathworks charges depends on your country and is technically a confidential offer.
