The Rear Brake Mastercylinder assembly is constructed from several parts from which the M/c Sub Assy,rr is the most expensive.
Some items from this specific schematic: Clamper,master Cy, M/c Sub Assy,rr, Rod Comp.,push, Pin Comp,brk Rod.
The lowest price component in this Rear Brake Mastercylinder parts-list fiche is the Screw-washer,4x12.