I think there is no correct answer . Your Mk5 to M6 at 7 and 3 years old seems about right to me .
I think it depends on what type of car it is , you could nearly get back what you paid for some cars in the past but they are an exception to the rule . Really sought after cars with a long waiting list can be chopped in after 6-12 months and you can get back what you paid and sometimes more , Ferraris and Morgans are an example of this .
I want to get a newer car than my ED30 but the Golf R i want to replace it with is too much extra to pay for not much more car (07 ED30 for a 10/11 R) I would pay £9 or £10k but they are more like £12 - £14k and thats too much for me . If someone wanted to trade down is about the only way i can see me doing it , they would get mine and cash . Have had a ad for a while on the R forum but no takers