Hi John,

There is no hard and fast rule on that one as there are so many factors at play, such as how badly the dealer wants to sell a car, the time of the month/year (many dealers will have targets to meet and may be more willing to get a deal over the line if they're short), demand for that type of car and, of course, the value of the car itself. 

It's a bit of a myth that the dealers operate on large margins though, so don't expect to get thousands off the price.