Killing Floor is currently $20 on Steam (not as cheap as the January sale I know, but still not that expensive), and I've heard some good things about it, plus it looks really fun. The only thing is, my internet connection isn't very good (wireless broadband/New Zealand internet in general FTL), so I wouldn't be able to play multiplayer at all. Would it be worth buying this game just for the single player?
Damn, yea, I like the idea of that flexibility with bots, so I probably wont get it unless there's a bigger sale on it again.
Also, I have looked into Mount & Blade because it's so cheap, but after watching some gameplay videos of combat I don't think I'd like it that much. Just seems pretty rough and not very satisfying somehow.
As for killing floor, for you its probably better to wait till its on discount.
As for Mount and Blade, I think videos dont capture the feel of the game. I would recommend trying out the demo. Its only 520 mbs. The demo is the same as the full version except there are limits in playtime and level you can reach. There is also a discussion about the sale on Mount and Blade forums.