Simple answer is that it depends, there is a huge variation in price and margin. Some dealers use autotraders own software tools to constantly adjust their prices to be competitive, some dealers slash their prices and do keen deals once they've been in stock a while. Some dealers will have massively overpaid on part exchange prices in Jan to try and ensure they hit their new car target and now have little room to move. Some dealers used car managers are only paid per unit sold, and some are paid a percentage of the profit.
Far too many variables to just say "get a grand off anything", unless you want a very specific spec you're better targeting the best priced cars first than chasing discount on the more expensive ones.