The component with the lowest price in the FRAME - SEAT schematic is the Plate,engine Mounting.
Parts such as the Frame - Seat: Tool Sea, Cap(od:17), Cushion, Lock Washer are included in the Rm400 1978 (c) Usa (e03) Frame - Seat parts fiche.
The Frame - Seat is made up of components including this Frame which is the most expensive.