The Department may contract with any of the counties in this State for the construction and improvement by any such county of any farm-to-market road in the State highway system within such county and may pay for the construction and improvement of such road, provided the cost of such construction and improvement is not greater than the cost of similar work would be if performed by contract or by the Department’s forces. Any road constructed or improved under the provisions hereof shall be paid for by the Department and the cost thereof charged to the allocation of the county in which such road is constructed. The roads constructed by the counties under the provisions hereof shall be constructed and built according to standards and specifications required by the Department. Nothing herein contained shall be construed to mean that the Department shall pay the cost of the construction or improvement of roads already constructed and improved.