At the time and place specified in the notice to contractors, the board of county commissioners shall examine all bids received and, without unnecessary delay, award the contract to the lowest and best bidder, who will perform the work and furnish the materials which have been selected, and perform all the conditions imposed by the board, as prescribed in the resolution for the work or improvement and notice for proposals. The aggregate amount of the contract shall not exceed the final estimate of cost submitted by the engineer for the improvement as provided in Section 1238 of this title. In the event of any excess in cost over said engineer's estimate, the excess shall be void and no assessments for such excess levied. The board shall have the right to award a contract for all or a portion of such improvement or to reject any or all bids, and to readvertise for other bids when any such bids are not, in its judgment, satisfactory. The letting of the bid shall not be complete until the contract is duly executed, and the bonds approved.