Who cares what most people think the car costs? That should be a completely irrelevant to your decision whether or not to buy a car. Brabus is not out to build cars to impress other people, but to impress the owners themselves.
Now personally, I do think $500K is too much money for what you get, but that is the cost of a micro-volume custom builder where each is hand built on order. There are no economies of scale. For me the problem is the car doesn't look classy. I would want deep glossy paint finish and a far less overdone body kit.
Eddy, while the four doors and four seats may take minute quantities off the extreme top end of track performance, there are few drivers in the world that would likely be able to notice or exploit the differences in the driver's seat. The added passenger and cargo abilities make a far greater impact on the total ownership experience than that last speck of potential track ability.
A truly great car should be able to haul more than just ass.