I bought CSS a (long) while ago for about £15.
It was the best game I've ever bought! Not only do I really enjoy HL2 (which came with it) and CSS, but also the extras, demos and mods that came for free (I am a fan of the simple games like peggle and trackmania). It's also pretty cool to use the SDK to make your own CSS maps, though a steep learning curve.
I currently have 13 items in the games list - having paid for just one! Awesome!
(and no, I don't work for valve lol)