I recently got my main battery replaced at the authorized Mercedes service centre and they charged me about $365 including labour. The new battery came with 2-year unlimited kms warranty. Interestingly, when I asked about the AUX battery, they said my car (220, 2017) did not have one. Daimler is using Voltage dip fuse instead for Eco Start/Stop function. More can be found in this article > "Ignition current limiter replaces the backup battery concept"