I got my Shimano Di2 in 2014, and there haven't been any maintenance costs due to the electronic components per se. I've replaced cassettes, chains, chainrings and cranksets, due to normal mechanical wear (and trying new gear ratios), but I haven't had a single expense related to electronic shifting, subsequent to purchase (apart from a few $0.01 added to my electricity bill).

I really beat on mine nearly every day, FWIW.