Green Man Gaming offers Steam downloads Killing Floor 2 for Windows for $20.09. Coupon code "SAVE20-NOVEMB-GAMING" cuts it to $16.08. That's the lowest price we could find by $4.
Also available are Steam downloads of Killing Floor 2 Deluxe Edition for Windows for $26.79 before coupon, $21.44 after. That's the lowest price we could find by $5.