The expense of such reconstruction, or of the building of a new bridge is payable from the same fund provided by law for the maintenance and repair of any such bridge. If the county makes such an agreement with any person for the building of a joint bridge, the county shall pay from such fund only its portion of the cost of the joint bridge, as settled by the agreement. In no event may the county pay more than one-half the expense of construction, repair or reconstruction of any such joint bridge.