I've ridden on 10, 11, and now 12-speed Di2 groupsets over tens of thousands of miles. The maintenance costs have been exactly $0 more than mechanical (okay there is the occasional charging, which adds a few cents to my electricity bill).

Hardly ever had to make any derailleur adjustments, never replaced a battery or a cable. In fact I've just pulled the 10-speed Ultegra Di2 off my older bike and it's operating as good as the day I bought it!