The state highways and transportation commission may, when in its opinion the best interests of the state require, contribute not to exceed thirty percent of the cost of construction of such bridge or bridges when constructed under the provisions of sections 234.210 and 234.220 and when such bridges form a part of or a continuation of the state highway system, and may incorporate any such toll bridge into the state highway system and maintain such bridge as a part of the state highway system; provided, however, that such law shall create no obligation or responsibility on the part of the state highways and transportation commission to make any refund for any moneys expended in the acquisition, construction, maintenance or operation of such bridge.