Whenever the plans for the construction of a trunk highway indicate that there will be a surplus of earth materials from the roadway excavation of such trunk highway, and a political subdivision or public agency of the state indicates that it has use for all or part of such surplus earth materials, the commissioner of transportation may enter into an agreement for the sale of such surplus materials on an equitable basis to such political subdivision or agency. Prior to such agreement the state in its proposal shall require a bid item reflecting either an increased or decreased cost to the state by reason of the proposed sale of such material. If such bid item indicates an increased cost to the state, the materials shall not be sold for less than such increased cost.