2013 R56S:
I knew it was expensive to have the battery replaced at a dealership, but holy crap! $543 total for diagnostics, battery and labor.
A couple days before I took the car in to get the right front scuttle lamp assembly replaced (the lens fell off somewhere and was lost, and you can't buy it separately), the display began showing the car-on-a-lift icon when started, plus a warning triangle that wouldn't go away. I asked the service rep. to have the warning lights checked out while the scuttle lamp was being replaced, and the answer came back: bad battery.
Oh well, at least I got eight (8) years out of that one. If this battery lasts that long, maybe it will be the next owner's problem.

 

Ouch. My diagnostics involved a sub $10 USB charger with a digital voltmeter (so I can keep an eye on my battery resting voltage, and the alternator output voltage). That, and listening for that first sign of the engine not turning over QUITE as fast as it should.

IIRC, the new battery was around $120 from Walmart (I like that option because they're good at keeping track of the age of the battery, which has produced some free or cheap warranty replacement - not to mention there are only a few battery manufacturers so no use paying more for one with a fancy sticker). ;-)