discount
It's kind of a "How long is a piece of string" question.
It depends if you are buying from a second hand car trader or a main dealer, if you are buying new or second hand and what make and modle you are looking at. You should look for at least a grand and a half off but if it's an open book type of sale then throws things as well.
If it's a high spec old modle car then push hard and bear in mind that most new cars have been sold for the year at this stage so garages have huge stocks of second hands and equally huge overdrafts.