I've actually been in that meeting where it was discussed about when someone would die. So I wouldn't be surprised. Either way, I can assure you it's part of the revenue forecast.
So if you are factoring the actors death into your budget when creating the movie you're either a total idiot or willing to predict:
- An instant-seamless distribution system, aka the internet
- Your company will be around 25-50 years from now
- Robin Williams will become an immensely popular actor
- This movie will be so good lots of people will want to watch it in 25 years
Edit:
I can see this being more likely in the music industry as I think people are much more likely to go back and listen to an old album: they see an artist die, they think, "I used to love that band" and they buy the album. With movies, until the digital era arrived, I don't think people did that as much.