Its hard to answer, but the easy one is that MB will price as high as the market allows them to.
But aside form that, MB spends a lot in developing new technology that needs to be funded and that will reflect in the car prices.
We actually checked the GL, MDX, Lexus GX and X5 before we decided on the ML, there are pros and cons on all of the ones we tested and for us the ML came out the best.
What I can remember about the MDX, was that it did not feel so solid as the ML, like when you close the doors the MDX feel light and the ML feel more like a vault in addition the interior was too busy with the touch screen the ML looked cleaner inside and the open ash wood in the ML looks and feels more luxurious than the lacquered wood in the MDX in other words the ML feels less plasticy inside.
Money wise there was almost no difference between the 2 we wanted, the MDX had the video screens behind, but the ML had 360 camera.
You will almost at anytime get more car for the money with a Japanese car compared to a German car.