The reason I think the G500 is more expensive to operate because of the initial buy in price, mpg, and premiums for parts/repairs. My insurance cost is nearly identical from my 2000 Montero to 2003 G500. I only have to change the every 10K miles on the G500 which save me some money when compared to the Montero. The car payment will be much larger than a Montero but it could turn out to be a good investment. I don't foresee the 2002+ G500's dropping below $20K in the next 10 years. They are much like the Toyota FJ80 Landcruisers, they will retain their value and possibly increase in value. I get a free rental car when I have repairs done at the dealer which is awesome. Last time I got a free rental car for a whole day because the mechanics were out to lunch when I took it in for an oil change. They just had me pick it up the next day. Mercedes service is second to none. I have had a few repairs but all of my repairs have been part of bringing the vehicle back to immaculate condition. I could hear the A/C hissing so it needed to be recharged. For a receiver dryer and a recharge it was about $600. Not too bad, in my opinion. Most report MPG's in the 13-14mpg range. I get between 15-19mpg but I think I am a rare case.
From my experience there are just a few trouble spots on the G500. They are listed below:
Tcase acuator
Window regulator motor
Front door pockets
Because all 3 of these issues can be resolved by going with an older imported G500/G320, many die hards look for the older G-wagons. Like I said, I like having the luxury like the heated steering wheel, heated seats front and rear, front and rear lockers (not all imported Gwagons had the front locker option), better dealer support, and more safety features.
No matter what you choose you can't go wrong. I have really enjoyed mine and it has been trouble free in my opinion. After all it is a 10 year old vehicle. My 10 year old Landcruisers were much less reliable.