Zak,
I have 1991 (manufacturing) and 1993 (registration) W463 SWB 300GE and 1998 G55 AMG. Prices went up recently in Europe. Go to mobile.de to see.
Maintenence: there is more, I think, not related to engine care, but you have 3 propeller shafts (instead of one), 3 differentials (instead of one) and two gear boxes. You also have the electro-hydraulic system of 3 differential locks. Earlier cars had cardans on propeller shafts, later had homokinetic joints, which were easier to maintain (not sure if I got the names correct in English).
I think this is the difference. You need to have a mechanic who knows these cars.
Some people may have this impression also because some cars are taken off-road - and this makes a big difference, as you can imagine.
Other than drive train and results of off-road - maintenance is similar to the other cars in my view. In MB dealership you will have higher price per hour than in some of the "flat" cars.