Play.com are currently offering the Pioneer HTP-GS1 Xbox 360 Surround Sound System for £149.99. The actual RRP for this surround sound set is £329.99 and the cheapest I've seen it elsewhere is about £240.
That said, it is currently showing as 'out of stock' but so it was on Sunday when I placed my order, and I received notification today that it has shipped so they are obviously still getting stock in.
In addition, Pioneer are running a promotional offer. Two XBox 360 games will be sent to you free of charge with Proof Of Purchase of this set before 31/01/2007 (the games are Burnout Revenge and Need For Speed : Most Wanted). The following link will take you to the form that needs to be completed and returned ...