The rear boot spoiler is around $700. It already comes painted and is color coded to your car colour. The other accessories worth looking into is the aero or ground effects kit which you can see in the Lexus US website. Melbourne Lexus are selling it for $3990 fitted. This includes the front lip, side skirts and rear lower lip. It does not include the rear boot spoiler though.
Sydney dealerships do not have the parts listed for some reason, but Parramatta Lexus is able to get them for you since they got mine.