'Retrieve the paid course mode for a course.
The returned course mode may or may not be expired.
Unexpired modes are preferred to expired modes.
Arguments:
course_key (CourseKey): The location of the course.
Returns:
CourseMode tuple'

def _get_paid_mode(self, course_key):

    (all_modes, unexpired_modes) = CourseMode.all_and_unexpired_modes_for_courses([course_key])
    for mode in unexpired_modes[course_key]:
        if ((mode.min_price > 0) and (not CourseMode.is_credit_mode(mode))):
            return mode
    for mode in all_modes[course_key]:
        if ((mode.min_price > 0) and (not CourseMode.is_credit_mode(mode))):
            return mode
    return None
