This is not always an option, but if possible, find yourself a local independent shop / mechanic.
Personally, I was fortunate enough to find one who has numerous years of experience working with Rovers, and charges a fraction of what the dealer charges. My maintenance costs therefore have been no higher than any other brand (if not lower).
But to take this one step further, my two Sports have been very reliable. Brakes, bushings, oil changes (stuff of this nature) are the only types of maintenance required and this in turn has helped with the financial aspect of owning a Rover.
I've owned several other brands that have not been as reliable, and therefore ended up costing more to maintain. Compare apples to apples; the cost to maintain a Rover maybe a little more, but be sure to also compare reliability.
Great Point RRS2006! We have a local Indy BMW Shop where all my BMW Owner Friends go, in fact, in most cases they are better than the Dealer. I took my Wife's Car in for a 6K Oil change and check-up. They are all Factory Trained Master Techs, and the thing I like is that they don't have an "Oil changing Kid" for the kind of service I wanted, the Oil change and check-up was performed by one of their Master Techs.
I've heard from Friends they are less expensive than the Dealer, but the most important thing to me is that they do it right, with OEM Parts and Fluids and don't forget the screw in the Oil Filter properly etc. They noticed a couple of scratches in the Paint that I knew were there and gave it to the Paint/Body shop next door while they had the Car and they polished them out for free!! I need to drop some Donuts off.
Im not sure about the new ml's... But before my sport i owned an older ml and put 100,000 miles on it with no problem... The mercedes service is a bit higher but i never really had too many major issues with mine. The range rover has more parts therefore more things to break. If i had the choice id go with the range rover all the way.
If you are talking about routine maintenance expect to pay more for the RRS. All else equal in the car world the bigger engined, heavier car will cost more. Case in point, my wife had a 1990 Miata when I met her. Sold it to a friend of mine 13 years ago who still has it. Still on the original clutch after 23 years and 180K, and has had 3 sets of F rotors and pads in that time. The rear rotors are still original.
V8 vs. V6 and RRS has 500# on the ML - more brakes, more tires, engine oil
If you hang on to the RRS long enough the added complexity of the suspension will add up as well.