Mathematically it would be simple, but I don't think it's as simple as just splitting based on time. They'd probably argue the song or movie clip added more value e.g. if I used a famous song in my 10 second introduction clip, that's a major branding move, is that still only worth 1/60 of the revenue from my 10 minute video? Or would I have many fewer viewers with weaker branding week on week?

There is also the right of the creator to have their work not used in ways they don't approve of. This a commonly heard about when politicians play songs at rallies. It's not just about the dollars.