in germany the D800 is about 600 euros cheaper then the 5D MK3.Whether one camera is worth more than another is very subjective and depends largelyt on what you shoot. If you look at the cameras in isolation and only shoot non-action, then the 5D MkIII certainly isn't worth 600 Euros more. However, if you also shoot action, then it is less clear cut. For me, the AF, frame rate and build quality, not to mention the fact I am use to Canon ergonomics, have Canon lenses and few other features are worth it. Would i prefer it is it was cheaper? Of course I would, but then I'd also prefer the D800 to be cheaper if I was a Nikon shooter, it's all about perspective and what is important to you as a photographer and consumer.
is the 5D MK3 worth 600 euro more?
in my opinion the answer is a clear: NO
Wow! Thank you for making this point! If the D800 was actually designed in the same vain as the mk3, and had the same function then yeah you can make the $600 more no. But, if you are in fact a working photographer and accept the factt hat a body is a tool then you have to acknowledge that yeah, you can use a screwdriver as a hammer in a pinch, but, a hammer does the job better. I know the OP said, lets not make this another d800 vs 5d3 thread, but, you really can't open a can of worms and still open the can of worms.
in whatever currency you use, is the price of a mk3 worth it? Like Kernuak said, the only reasonable answer is --- "subjective and depends largely on what you shoot." The d800 is strongest in the lower ISO range, in the studio it's a beast, and if you shoot landscapes and print big, then yeah, it's obviously a better choice (unless you shoot architecture, where the controls on tilt shift lenses are blocked by the protrusion of the built in flash). The mk3 shines in the higher ISO's, which is preferable to sports , wedding, and event shooters. the mk3 has a larger buffer, which yeah, only matters subjectively. If you shoot sports or weddings, a larger buffer is a good thing, no landscape photographer is worried about that though, nor would frame rate really be a concern - so yes----it's subjective. file size is another issue - if you shoot weddings, even if you have a computer that can handle the workflow - you need more memory cards, and more backup storage. IMO - the d800 can do pretty well in low light, so it could be an attractive camera for wedding work, if you had multiple RAW file sizes to choose from (like how canon offers Raw, mRAW, and sRAW. Yeah you can set it crop mode, but whats the point of having a FF body if you have to shoot cropped just to reduce the file size/resolution? But back to the point, this is a subjective need. Again, the d800 is more of a studio camera. You aren't shooting as much in the studio as you do at a typical wedding.
Just some food for thought!