The reason the BMW dealer will charge so much is they're only meant to use genuine BMW parts and in this case that means an endless chain so they'd have to take the swing arm off to fit it. Quite a few will fit on BMW parts if you ask them nicely and supply them yourself. We'd be just as happy to get the independent to fit a non-BMW chain as long as it uses a soft link not a split link and it's from a well known manufacturer (DID, Afam, Regina, Tsubasi). The only potential problem is if it does break and damage the motorcycle BMW warranty won't cover the damage but if it's fitted and maintained correctly this is extremely unlikely.
Failing that you could fit them yourself as it is a reasonably straight-forward piece of motorcycle maintenance, so long as you have the right tools. We ran a article on fitting chains and sprockets about a year ago ….Changing the Chain of your Motorcycle...which should help you through the process.