I have a couple cars, some german. It absolutely does not cost $700 to change a dead battery any more that it costs $100 for a bottle of water. The cost of goods is cheap. You do need to tell the computer it is a new battery, since it adjusts charging voltages based on battery age, and if you don't do this it may overcharge the battery.

Talking to the onboard computer is free beyond the initial equipment and it isn't even locked or anything, tools that can do this can be had for cheap, connect to your phone via bluetooth etc.