I have to say I was pleasantly surprised with what I was offered for pt/ex recently. Is200 Sport 2001 Y plate, black, FLSH and 89k on the clock. Autotrader, adjusting for the mileage, suggested about £7k. Managed to get BMW Glasgow up to £8.5k.
Then again, I may not have been getting a great deal on the car I was looking to buy. Give with one hand, take with the other I guess...