Apologies to anyone who may have posted this already, and for the general cack-handedness of the post, (it's my first!), but I just found the expansion pack for World of Warcraft for £9.99.
As far as I know, the next cheapest I've seen this is £17.99 at a variety of online and offline stores.
This is part of the latest sale at play.com.
Hope you find it useful!