Interestingly, i have just come from a thread with a potential buyer for a ML63 AMG asking for running cost advice, and as per a few threads, someone dived in with the '£4000 a year services, tyres, pads discs etc' dissuader...the guy musta ran a mile after that!
As stated, i run a ML AMG also.
If like me, you buy the best example available, treat your investment with respect, maintain on the button, the running my AMG's has been cheap as chips.
I have an account with my local main dealer, all work is carried out there, and you will be amazed at how much you can negotiate on prices for labour with customer loyalty and striking a good relationship with the service manager.
Alternatively, buy a 'cheap' AMG, and you may just break your heart. Most performance cars with high amount of owners usually have been thrashed and then flipped before any maintenance bills, and if that pattern continues, someone, somewhere ends up paying big along the line.
I tend to try and buy privately also, as you get a good feel for how a car has been looked after by assesing the seller; you don't get that with dealers, and a folder of servicing sometimes doesn't give you the big picture either.
As a footnote, my E55 runs and average of maybe 23.5mpg on approx 5k miles a year; however, on a 1300mile trip the other year with 5 of us, and all luggage for 2 weeks away, it returned 32mpg, which i thought was great. Basic 'A' sevices approx £200, basic 'B' approx £300, anything requiring over and above that gets the price negotated. I don't rag it, so tyres seem to last about 2 years on my mileage; this includes the fronts as they start to wear with 'dry' turning whilst parking with the weight of the engine. Pads and discs seem to go on and on; i thought they may require a replacement last service, but MB didnt reccomend it, so, fine.
This isn't a plug for my E55, but just to help anyone wanting to buy an AMG that gets put of by people whom may have never ran ones' ill-informed claims like with the ML63 thread. My ML55 AMG is exactly the same; has been very cheap to run sevicing and maintenance wise, fuel economy is poor, but really, everyone whom buys any car with over 300bhp knows fuel will be a factor; however, most ML55 threads get hijacked by individuals that probably have never even sat in one but still post how crap they are in every sense, and yet, i cant praise my 5 years of ownership of my ML55 enough.
In a nutshell, always buy the best example available, and it should pay you back in spades. Running costs will be determined by your purchase choices and driving style.