BMW is priced higher than Audi and MB, but IMO the 6GC looks a lot better too! I used to own the 2012 Mercedes CLS550 and I loved it. I wish I never got rid of the car honestly, but that's another story. Now I own a 2013 640i GC and my wife says she "hands down" thinks it's better than the CLS550 even though the CLS is a V8. It looks better, drives smoother, and the interior is way better. Also, most people are getting great deals on the 6GC. I got 10k off my car which made it less than the CLS I previously owned and about 3k more than the A7 I priced (all the cars were discounted). Also, I would've received an even bigger discount on the 650iGC but I opted for better gas milage.
I've owned Audi's, BMW's, and Mercedes and I think all of them make great cars but, as for the four-door coupe I think the 6GC has the wow factor. Great performance and superior styling!! Also, if you can get 10k off it's a no brainer (atleast for me).