The Nut,hex 6mm is the lowest priced item on this Rear Wheel schematic.
Parts such as the Rear Wheel: Nut,special,6mm, Wheel Sub Assy,rr, Bolt,sprkt.fix., Adjuster,sr.chain are included in the Crf50f 2008 (8) Australia Rear Wheel parts fiche.
The Rear Wheel assembly is constructed from several parts from which the Wheel Sub Assy,rr is the most expensive.