The reason the 4200 is getting costly to maintain is age related, they are all getting on a bit and parts are getting harder to source. They also need specialist care, I know some of you maintain them but for most of us they are too complicated. I found my 4200 to be the mist expensive car to run ever, even more than my Supercharged Range Rover. OK I had some bad luck gearbox actuator etc but other stuff clutch, heater matrix, wishbones are all common failures on the cars these days and each will sit you back a significant chunk of cash.
That said the same is probably true of similar age fast BMWs, Audis etc never mind comparable sports cars Porsches, Ferraris et al