When any plan shall be approved by the county commission, the superintendent shall immediately advertise for bids for the erection and construction of same, stating in such advertisement a description of such building or buildings, and shall contract with the person or firm who will agree to do the work and furnish the necessary material on the lowest and best terms not exceeding the amount appropriated or set apart for such building or buildings; provided, that in case the lowest bid received shall be in excess of the amount appropriated, after two successive advertisements for such bids, then, if such county commission shall be of the opinion that such building or buildings can be constructed or built for a sum not in excess of the amount appropriated, or if the commission shall be of the opinion that there is collusion or combination between the bidders for the purpose of forcing the county to pay an exorbitant price for the construction of such building or buildings, then, the commission may, in its discretion, let such building or buildings at private contract, to be constructed according to original plans and specifications upon which bids were received and approved by the county commission.