Not the cheapest this has been but the cheapest i can find at the moment.
The codes been going around a bit today, sign up here http://rewardyourthirst.com/Register.aspx
, Recommend 3 friends (make up 3 fake email addresses, it does work) then redeem you 15% zavvi discount) takes 2 minutes.
Obviously you can use the discount on whatever you like but i thought it made avatar 3 disc collectors edition a much better deal! =D