Yesterday I traded in my old 20GB Xbox 360 (one with HDMI) along with an extra controller and the wireless adapter and got a new 250GB with Halo Reach for only £82.98. Thought it was a good deal personally. Cost breakdown as follows:
New Xbox 360 Slim 250GB £149.98
Halo Reach £24.98
Xbox 360 20GB -£50
Extra Wireless Controller -£12
Wireless adapter -£15
360 + 1 game promotion -£14.98
I believe the games you could have gotten on the promotion were Halo Reach, Forza Motorsport 3 or Fable 3.
It was actually cheaper to get a new one then buying a second hand model which I was planning on doing.