I would like to add one piece of info to this discussion.
I see a lot of comments along the lines of... "why pay the dealer $400 when I can get a new battery for $90!!"
Modern BMWs come with a absorbed glass mat (AGM) battery that is far better quality than the old style lead acid battery. AGM batteries cost quite a bit more than lead acid batteries, so if we are going to make this about price we should compare like batteries.
A little Google-Fu turned up this
http://www.odysseybatteries.com/battery/pc1220.htm link for a group 94R AGM battery that will fit a new 3-series. As you can see the battery is $268. If you go to the dealer you will be getting a AGM battery. Not really fair to compare that to the $90 lead acid battery from Walmart.
If you factor in the cost of an equivalent AGM battery, the dealer prices we are seeing quoted in this thread do not seem to unreasonable when you add in labor to R&R the battery, plus hooking up the computer to do the registration.
I'm not normally one to defend dealer prices, but I thought this was important enough to be pointed out.