Council may authorize a contract with a single or general contractor, in lieu of general, mechanical, electrical or other contractors, for the construction of public buildings or facilities having a gross cost not in excess of one hundred thousand dollars ($100,000), when, at its discretion, it determines that a more efficient and less expensive performance of such construction shall be had. Council may also determine that supervision for work costing less than such amount will be better administered by its agents, in which event, supervision will be performed by City employees designated by the Mayor.