The board shall then proceed to advertise for bids and let the contract for the improvement or construction of such highway after first securing the department’s written approval of the proposal submitted by the successful bidder. If the bids for improving or constructing such highway exceed a figure which the department considers reasonable, such bids shall be rejected. In such case the project may be readvertised and if, upon such readvertisement, no satisfactory bid is received the work proposed may, subject to the approval and under the inspection of the department, be undertaken by day labor.