Currently £19.99 on steam, but this does the same job (am 99% sure you don't need the disc in, as it will all work via steam once you get it).
This includes access to all of the DLC including The Passing and the weekly mutations. You can also install maps from the likes of http://www.l4dmaps.com/
, or make your own using the authoring tools supplied with the PC version.
Don't forget quidco/topcashback!