Except for professional education courses, there is no cost for taking edX courses when you enroll in the audit track, which does not offer certificates or provide graded assignments.
However, if you want to be able to earn a certificate for a course and complete assignments, there is a fee that will vary depending on the course. The fees for the verified track usually range between $50 USD and $300 USD.
The fees for upgrading to the verified track or enrolling in a professional education course are the only charges on edX. There are no other fees associated with edX courses.
You can find more information about the different certificates offered by edX: What types of certificates does edX offer?
edX has also introduced a financial assistance program to help learners who demonstrate financial need to earn verified certificates at a reduced cost.