Valve is running a LucasArts-specific promotion on Steam all week long, which means that you can nab a bunch of Star Wars PC games for cheap. Included among them is Star Wars: Knights of the Old Republic, which is only $2.50 for the next several hours. If that isn't enough, you can grab the entire Star Wars collection for only $49.99.
Why Star Wars: KotOR II isn't on Steam yet, though, is beyond me.