'Returns the minimum price of the course in the appropriate currency over all the course\'s
non-expired modes.
If there is no mode found, will return the price of DEFAULT_MODE, which is 0'

@classmethod
def min_course_price_for_currency(cls, course_id, currency):

    modes = cls.modes_for_course(course_id)
    return min((mode.min_price for mode in modes if (mode.currency.lower() == currency.lower())))
