The G35 is really a parts-bin car from Nissan stock, much like the low/midrange Lexus are just Toyotas in fancier trim. Maintenance is no worse than a Maxima or any other VQ Nissan. Of course if you get one with 19" or 20" wheels, then your tires are gonna be pricey. The VQ itself is really robust and doesn't require timing service either.
I had an '05 G35 Sedan for work, I got about 22 mixed, and could see nearly 30 on the freeway if I kept it under 70.
Now the German luxury brands for the most part are a bit more expensive to maintain. A lot of this has to do with their target market, and the fact that particularly with BMW and Mercedes, there aren't any cheap models that share their parts bin. So you look at something simple like a radiator hose or battery, and chances are it will be 50-100% more expensive than an equivalent part on a Toyota/Lexus or Nissan/Infiniti, and ditto Honda/Acura.
The G35s are very strangely priced. The sedans imho are one of the absolute best buys in the used market due to being proven extremely reliable and cheap to maintain, while not costing very much to buy. Hell, compare the price of an '06 Accord V6 to an '06 G35 Sedan, it's stunningly close. But if you look at the coupe prices, things change dramatically. There's nothing special about the coupe other than losing two doors (same drivetrain, basically same weight), but they usually cost a ton more. Maybe it's a Nissan thing, as the Altima coupes also demand a huge premium over the sedans.