Several friends of Irwin Allen who signed on as investors made out like bandits despite the fact that they put zero money into the film.
You've seen Guy Standeven in something because the man was in everything.

And I suppose some of the actors had contracts based on gross earnings, too.
oh, I don't know, especially when it wasn't expected to be a hit (as you said) With The Towering Inferno, I could see that happening.
However, if I was the producer, I would pay them something extra retroactively if it earned that much, regardless of the contract. I dont think the cast of
Night of the Living Dead
received a bonus after it earned 100X it's budget. My feeling is is that just because you don't HAVE to pay someone a bonus, doesn't mean you have to avail yourself

I read the director or whoever used some of his money to get this movie going. The studio was only going to give them so much or something. They didn't think people would be interested in this kind of movie. Maybe I'm thinking of the wrong movie lol.