The component with the lowest price in the OIL TANK schematic is the Tube-pvc,5x440.
Parts such as the Oil Tank: Cap-oil,tank, Tube-pvc,5x440, Screw,6x16, Tank-oil are included in the Kmx125-b10 1999 United Kingdom Fr Fg Oil Tank parts fiche.
The Oil Tank is made up of components including this Tank-oil which is the most expensive.