I spend far more in toys than I do to keep my baby running.
I am a DIYer so the only time the car sees a shop is for stuff too heavy or if I don't have time. So I have only spent a few hundred $ on maintenance.
If you are at the mercy of the dealer, Yes they are expensive to maintain. But then so is an E32 7 series which is about 60% or so common.
The electronics are quite good and stable but the dealers seem to be computer swappers for even simple problems. $$ add up quickly that way.
I have a good independent for the heavy stuff though. I can tell them what it needs and that is where they go first
They don't change the fuel pump, fuel pump relay and the ECU because the less than $100 crank position sensor is bad. (The failure mode is: If the CPS is Open, the ECU disables the fuel system by removing the ground from the fuel pump relay. Nobody ever looks for a missing Ground
)
So if you are DIY inclined they are simple and not much more expensive than any 3 series to fix. There are sites where a lot of the potential repairs are photo documented from end to end.