RRP £14.99, seems to the cheapest Official Sony one at the moment:
The Hut £9.93
Game Collection £11.99
The Playstation Move Gun Attachment allows you to use your Playstation Move Motion Controller in a more natural fashion when playing shooting games compatible with the Playstation Move system.
*Playstation Move Controller Sold Seperately
Should be good for games like RE5 Gold and the upcoming Time Crisis, KZ3 and Socom games.
Don't forget the Quidco / TCB, and any Hut vouchers you've been sent!