(1) Except as provided in section 23-3109, purchases of personal property or services by the county board or purchasing agent shall be made:
(a) Through the competitive sealed bidding process prescribed in section 23-3111 if the estimated value of the purchase is twenty thousand dollars or more;
(b) By securing and recording at least three informal bids, if practicable, if the estimated value of the purchase is equal to or exceeds five thousand dollars, but is less than twenty thousand dollars; or
(c) By purchasing in the open market if the estimated value of the purchase is less than five thousand dollars, subject to section 23-3112. In any county having a population of less than one hundred thousand inhabitants and in which the county board has not appointed a purchasing agent pursuant to section 23-3105, all elected officials are hereby authorized to make purchases with an estimated value less than five thousand dollars.
(2) In no case shall a purchase made pursuant to subdivision (1)(a), (b), or (c) of this section be divided to produce several purchases which are of an estimated value below that established in the relevant subdivision.
(3) All contracts and leases shall be approved as to form by the county attorney, and a copy of each long-term contract or lease shall be filed with the county clerk.