Looking for a listing of EdX courses?
Most edX courses are available to audit for free but certificates are not available without paying.
Individual courses, Professional Certificate Programs, Professional Education, MicroMasters and XSeries cost varying amounts. Prices are available on the edX information page about each course or series of courses. You can find an explanation of the different types of qualification here: https://www.classcentral.com/report/edx-professional-certificate/
Students who are unable to pay can apply for financial aid: https://support.edx.org/hc/en-us/sections/115004147627-Financial-Assistance