If you to stick with the ~$400-600 range, the Samyang 35mm, Canon 50mm f/1.4 (or if you can stand the tiny focus ring 50mm f/1.8 for $100), 85mm f/1.8, 40mm f/2.8, 35mm f/2.0, 24mm f/2.8 would all be good lenses. They are also all EF mounts, so they'll work on a FF if you want to move up to that from a crop sensor later on.
As fantastic a value for money as the 50mm f/1.8 is, I can't recommend it for video. The focus is just too fiddly. Tension on the focus ring is nonexistent in MF and tiny movements have very large effects on focus. To be honest even in photography situations, with magnified live view, it takes a lot of very delicate tweaking to get the focus just right.
As for the Samyang 35mm I don't own it myself but the video I've seen from it looks great, and of course its primary drawback--the lack of autofocus--is a complete non-issue for video.