$153 for JUST the backrest....then another $70 for the pad. That is just ridiculous...why don't these all come as one piece!?
I have found the cobra standard/tall (it is 17 inches) backrest to be the most cost effective, and comfortable solution to date. It supports far enough up the back to make long days in the passenger seat comfortable, and it is the least expensive I have found over all and comes as a complete unit, side bars, backrest and pad and hardware to bolt it on.
I would caution against getting a 'short' backrest even tho it might look more stylish. trust me, as the one who usually spends the time experiencing it, she will want the tall one. also, tall one is easier to hang sissybar bags off of for luggage.
if you are using the stock seat on your bike, she WILL want an airhawk pillon pad...
these suggestions are from one who has spent about 16k miles riding the passenger seat
before I started riding my own. 12k on my old virago, and the other 4k at various times when we had one bike in for service or for fuel efficiency just decided to use one bike not two.