That price seems fair to me. And, normally the shop will require they take the whole bike in for warranty - don't ask me why but you can always ask.
IF it is the side seal (which you hint it is not) it is very easy to do and can be done without removing anything but the plastic cover. Jim VB has a tutorial on it I think.
IF it is the pinion seal from inside the boot you will need specialized tools to do it - a machined socket to remove the collar on the pinion, air tools, heat and proper pullers - most likely going to have to fabricate some work-holding tools.
I have done it twice now - it gets easier but is not a simple job at all and you will spend more time machining the proper tools (it took me two days to machine the socket) and or buying them from BMW than the 375 is worth in my opinion.
It is a big job - really and if you are going to do that then you might as well replace the small pinion bearing or all of them at the same time as that bearing is the one that fails - then you are doing the shims and, and, and.... back lash and contact pattern/patch...
I have an 06, 120GS with 100,000 on it.
As your attorney, I advise you to buy a motorcycle. How else can we cover a thing like this righteously?