Whenever any improvement shall be ordered, payment for which shall be made in part from assessments against property specially benefited, not more than fifty percent of the cost thereof shall ever be borne by the entire port district, nor shall any sum be contributed by it to any improvement acquired or constructed with or by any other body, exceed [exceeding] such amount, unless a majority vote of the electors of the port district shall consent to or ratify the making of such expenditure.