The director may establish minimum prices to be paid by handlers to producer-handlers for milk not used by the purchasing handler as class 1 milk. These provisions may provide that the milk, if used in classes other than class 1 by the purchasing handler, may be paid for at the minimum prices established by the director for this other usage but which shall not be less than the prices as found by the director to be paid by manufacturing milk plants in, or adjacent to, the area that use milk for similar purposes. The prices shall remain in effect only for the period during which, as determined by the director, there is a surplus of producer-handler milk.