(a) Before the board of any park district created under K.S.A. 19-2859 to 19-2880, inclusive, and amendments thereto, shall let any contract for any improvement which is estimated to exceed $20,000, the board shall cause accurate detailed plans and specifications therefor, together with a detailed estimate, of the cost of same, to be made and filed in the office of the secretary of such board. Before letting such contract, the board shall advertise for bids to do such work in accordance with such plans and specifications for at least one week in a newspaper of general circulation in such district. Except as provided by subsection (b), the purchase of materials, contracts for purchase or sale, lease contracts and other contractual services which are estimated to exceed $20,000, shall be made upon competitive bids. All bids shall be made in writing and signed by the bidder. All bids shall be submitted or delivered by the bidder, or the bidder's agent or attorney, to the board or to a designated representative of the board as specified in the bid notice. The proceedings to open and consider bids shall be conducted at a time and place specified in the bid notice. Such proceedings shall be open to the public. All bids shall be accepted or rejected by the board at a meeting thereof within 30 days of the opening of the bids. The board may reject any bids and shall not accept a bid in excess of the estimated cost of the work, and a contract let at a price in excess of the estimated cost of the work shall be void.