Steam have a sale currently on Killing Floor as part of their Midweek Madness. You can now get it for 85% off.
From Steam “In celebration of passing several milestones like selling 1 million copies, having over 45 million hours of playtime and destroying over 500 million Zeds, save 85% on Killing Floor during this week’s Midweek Madness.”
You can get Killing Floor and the Killing Floor Bundle both for 85% off. The Killing Floor Bundle is the one to get as it has all the DLC included for only a bit more money. If you don’t care about any of the DLC, you can get Killing Floor by itself for a few pounds.
My other Killing Floor posts: