Backrests aren't cheap. Even a Givi made for a scooter, nothing but thin stamped steel and molded plastic will cost around $90+. Pirates-Lair has a really high quality unit with a "fabulous" luggage rack
That costs $395. Add a $40 accessory and his rack can then accommodate a trunk, just not in conjunction with the back-rest. Link below. Most people go with a trunk and the passenger can lean on it if needed. My wife wouldn't ride my bike until I did that.
http://pirateslair.net/ConcoursStealth.htm