The contracting municipalities may levy taxes for health and sanitation purposes and to pay a portion of the principal and interest on bonds issued under the authority of Kan. Stat. Ann. § 12-1774, and amendments thereto, by cities located in the county as may be provided by law except that the amounts to be budgeted for such purposes by each contracting municipality shall be fixed by agreement after taking into consideration the population of each and any other factors which would necessarily increase or diminish the costs to be borne by it in the absence of agreement to establish such joint board of health.