The deal I've found, looking good. ;)
For the deal followings need to be done;
1. Buy an Xbox 360 4gb Slim from Argos for £140.00.
2. From the additional deals when you buy an Xbox 360 from Argos choose the half-price Mafia II (£17.50).
This adds up to £157.50, but when you trade in Mafia II in CEX you get for it £28.
£157.50 - £28 = £129.50