I was comparing a two year old car to a two year old car. Depreciation is "non-linear" to be sure, but I'm comparing two cars over two years of ownership, admittedly with a bit of a difference in mileage.
If you were to say that the average car lost 50% of its value over the first 3 years of ownership, a $20,000 car would be worth $10,000 and an $80,000 dollar car would be worth $40,000. The amount "lost" to depreciation would be larger, but the depreciation of the vehicle would be the same.
I suggest you try and look at how some Toyota's hold value for resale. I suggest you try and look at how some Mercedes vehicles hold value for resale.
I have a friend whose parents picked up a 5 year old used Mercedes Benz S500 for $50,000 a few years back. New, in Canadian, it had been $135,000. It had very low kilometers on it (30,000 or so, over 5 years), and was in almost perfect condition. Got it from a dealership, too.
Depreciation is often less about the price of the original person and more about the reputation of the brand.
Here's an interesting article on depreciation by brand:
http://www.allbusiness.com/finance/868133-1.html