The city commission may by ordinance establish the maximum amount of funds which the chief executive officer shall be authorized to expend for budgeted items and establish the minimum amount of funds which the chief executive officer shall be authorized to expend for budgeted items which must be let on the basis of competitive bids; provided, however, all expenditures or purchases in excess of the amount established by state law must be let on the basis of competitive bids. The city commission may, by ordinance, establish a figure above which all contracts or purchases must be approved in advance by the city commission. All contracts and purchases shall be handled in a manner to obtain the best value for the city.