Mercedes is not expensive to maintain, but when you start talking about AMG, then that is a different story - it is big bucks and do not try anyone outside the agency for maintenance.  I've had several AMG's and seriously you will not get proper service and repairs done outside the agency.  The AMG I got now had been serviced outside agency and it has cost me lot's of money - actually nearly 30 K just to repair what an outside workshop have damaged.  Remember that the Engine is handbuilt, the Brakes are "racing"brakes..  Expensive..

And if you want to buy a used one, call NBK and ask for the service history.  And if you are getting a Cheap one, hmm I say it might not be what you think and hope for..