The component with the lowest price in the LEFT CRANKCASE COVER ALTERNATOR schematic is the Screw Cross 6x18.
Parts such as the Left Crankcase Cover Alternator: O-ring,13.8x2.5, Bolt,flange,6x25, O-ring,27x2, Stator Comp. are included in the Xl250s 1979 (z) Usa Left Crankcase Cover Alternator parts fiche.
The Left Crankcase Cover Alternator assembly is constructed from several parts from which the A C Generator is the most expensive.