The rack is a bespoke device that only fits one bike, hence the cost. It will come with a top plate to suit. You can possibly pick a cheap top box to fit on eBay just be aware that Givi have a few different types of top plate which require different top boxes and your rack will only support one type.
Have a look here:
http://www.givi.it/My-motorcycle/?ma=KA ... 600-93--01The reason for the cost is that the quality is top-notch and they are 100% waterproof.
A minor downside is that the instructions are pretty ordinary. I have come across this before with machinery built in Italy. The Italians generally speaking have a great flair for design and their engineering can be spot-on but they have an annoying habit of leaving minor finer details (like instructions) a bit lacking

. I also find the Japanese do this a bit, I put a factory screen on my Hornet and while the product itself is typical Japanese the instructions were a series of diagrams on a piece of paper the size of a postcard...wtf is with that, is paper expensive in Japan?
BTW while the rack on my Hornet didn't require anything special in the way of fitment (it mounted using the exhaust mounting bolts), my old ZZR required a couple of holes drilled in the rear ducktail subframe. They can't be seen and it's a non-structural area rear of the pillion, but keep it in mind.
thks, Mick