ur asking about the bushings right (the ones that the lower arm-to-chasis bolt goes through)? not the lower ball joints?
as for the rear, why doseth it needeth to be rebuilt(eth) ?
but to give you an estimate for the rear axle, the "rebuilid" kit (bearings, seals, etc..) usually costs about a hundred.. as for the labor, thatll cost u probably another $200-$300.