In making purchases, the commissioner of administration shall give preference to all commodities manufactured, mined, produced or grown within the state of Missouri and to all firms, corporations or individuals doing business as Missouri firms, corporations or individuals, when quality is equal or better and delivered price is the same or less.

(RSMo 1939 § 14600, A.L. 1945 p. 1428 § 74, A.L. 1965 p. 144)