It depends on the dealer, the car, how long they've had it and how close to this months sales target they are so it's a tough one to answer definitively I'm afraid. In my experience trade in offers are always exceptionally low, usually lower than you ever think they could be...they will come up a bit but will drag it right out and as before, it depends.
As an example, I was offered £17,000 by the dealer for my 04 M3 against the Z4M (advertised at £35,000, this was 2007). I ended up selling it privately A week later for £24,000....and got £3k knocked off the Z4M. Another dealer wouldn't give me more than £600 off their identical Z4M, and that one was priced even higher than the one I bought.
How long is a piece of string, basically....
On a side note, webuyanycar.com should be your absolute worst case baseline, their values are also extremely low (for obvious reasons....they buy any car)