Video is interesting in that it accelerates your efforts in other areas. The time you spend making a better video should also help you reach a bigger audience (A), increase conversion thus making advertising (D) more effective, it probably helps with PR too (E) since media outlets prefer to show interesting videos, and with page quality (C) to a degree since you can reuse frames from the video for the page.
So back to your question! how much to spend on video is you have $20k budget overall. From an execution standpoint, we certainly see a sweetspot ~$3-6k for video, in that it allows you to get a clean professional video that looks legit. That's enough to hire a director with a small crew for a day and simple editing. If you spend less then you'll have to take on some of the director's responsibilities to make sure you hit on the requirements we found in the benchmark (location scouting, quality images, tight narration...) We wrote more about this here https://www.videopixie.com/how-much-does-a-kickstarter-video...