I don't know what the exact numbers and ratios are, but when I was looking for a "new" car, I looked at used cars, too.
I was looking for something not TOO old, maybe 5 years old at the most, hopefully a little newer.
I thought (for some reason, not sure why) that Lexus would be a better used car buy than BMW or MB, but I was actually quite surprised at how much used Lexus cars cost. In my admittedly unscientific survey of used cars over a period of a few months, I was left with the impression that Lexus actually holds up its value as well (or actually even better) than BMW or Lexus. I couldn't find a good deal (from a buyer's perspective) on a used one.