All contracts shall provide that if any person who furnishes adopted textbooks to the State shall sell the same textbooks or cause them to be sold to any other person, state or state board for a price less than that which this State has contracted to pay for such textbooks, then such lower price shall automatically become the contract price for such textbooks in this State and the contents of the textbooks shall be considered and not the title in investigating such prices. The State Board of Education shall make the necessary investigations as to the prices of such textbooks so sold to other persons, states or state boards.