I have maintained since new a 2005 A8L, with the 4.2L gasoline V8, for a customer of mine. It has never seen the dealer once after it was purchased. It has proven to be a very nice, well constructed car. Solid and pleasant to drive. It has had a few things need attention, most notably the air suspension, which is hardly unique to that car. ANY car that has air suspension is going to sooner or later require air spring replacement. It has also needed two replacement sets of upper links, as their bushings are the same as the much smaller cars (so they simply do not last as long). It has also needed the nav screen motor replaced, which as I recall was a pretty pricey piece.
Tires are expensive, oil changes are expensive (seriously, you have to remove the air cleaner assembly to access the oil filter
), headlamp bulbs are expensive (HIDs, and you have to R&R the front bumper assembly, complete with its 20-pin wire harness connector), and it of course drinks premium gasoline like the owner has his own oil well in his back yard.
BUT... it is a very nice car, and I think it was well over twice what my new Passat cost in 2004, and my Passat was as expensive as you could have gotten without getting a GLX or W8 model. But the owner loves his Audi, and he treats it well. And it probably has cost no more to operate than a T'reg, and it drives INFINITELY better and doesn't creak or rattle over bumps in the cold and is extremely comfortable to drive.
So if you could get that same level of car with a diesel engine that barely uses any more fuel than a Jetta? Yeah, that would be pretty sweet!