Insurance is pricey generally for the reasons given by Grazy.
For a Brabus, the underwriters know how fast it is, they know it will be expensive to fix. They also know that almost everyone who buys one will at some time or another, drive it pretty rapidly.
I am sure, if I did a low milage and ran it as a second car, then I could insure it cheaply. I expect plenty of people do.
I am running it as my every day hack, and therefore it costs more for me.
I got my Brabus very cheaply, and I was very lucky in that respect.
Compared to a "shopping" forfour, it is not cheap to run. For a performance car, it is.
I say that comparing it to my last "every day hack" which was a subaru, which I loved. However, the Brabus is much cheaper to run (uses less fuel, and the Subaru needed servicing far more often, and ate tyres / brakes faster than the Brabus).
Second hand car prices are supply /demand and market forces will dictate what they are worth. The Brabus may be held back a bit, because not many people know about them. To be honest, it only matters if you are trying to sell one and were under the impression that it was worth a lot more than it is. On the other hand, if you want to buy a performance car and need rear seats, they are great value for money.