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. However, if you want to earn a verified certificate for a course, 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 enrollment tracks in the Enrollment Tracks topic in the edX Learner's Guide.