When the prisoners of a road camp are engaged in the construction or maintenance of any portion of the state highway the expense of maintaining them together with the compensation of such prisoners fixed by the board of directors as provided in this article, and the expense of supervision and maintenance of the road camp and the prisoners thereof, shall be paid for by the district and the State Department of Public Works upon such terms and in such proportions as may be agreed upon by the Department of Public Works and the district.
Ca. Pen. Code § 4216