I managed to buy a matching top box to complete the Tiger adventure luggage for $AU300 and was fairly pleased with myself until I found the mounting plate to bolt it to the bike is $AU290! The bloke at the Triumph shop said it's because they have shock absorbers in them. Just as well, because I was shocked. After looking at ways of building one I buckled and paid up as the alternative was to look at the bargain top box sitting in the shed.
It's an absolute disgrace the price of accessories. but they know you want / need them so you've got to pay. It's like a got you by the bollox tax.