Of all the components used in this assembly the Caliper Assy is the most expensive.
Some items from this specific schematic: Bolt, Caliper Support (3g1), Caliper Seal Kit, Nut, Hexagon (509), Boot, Caliper.
The most reasonably priced component used on the Xs650s 1979 Usa Rear Disc Brake-caliper is the Washer(526).