For the purposes of paying its share of the cost of the purchase, improvement, support and maintenance of such detention home or juvenile farm as determined by the agreement for the joint undertaking 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, the board of county commissioners of each county party to the agreement shall levy an annual tax on all the assessed taxable tangible property of the county.