I was also looking for such information a year or so ago. I worked out my own very rough rule-of-thumb was that cars depreciate by about 50% every three years.
Example:
New car: 20,000
Three-years old: 10,000
Six-years old: 5,000
Nine-years old: 2,500
However, I cannot understand why the actual factual information isn't published - it is in other countries.