Prisoners employed under section 30:8-40 of this title shall be under the custody and control of the board of chosen freeholders who shall make rules and regulations governing their employment and employ necessary guards and attendants while prisoners are so employed and fix their duties, salaries and terms of employment. The compensation of guards and attendants, the wages or earnings paid to prisoners as hereinafter provided and other necessary expenses in connection with their employment shall be included in and charged against the cost of such work.