I bet the shooting schedule was so tight that it didn't consume a whole lot of the actors' time. Once they had a few on board it became an "everybody who is anybody is doing it" so people wanted in at whatever rate they were getting.
I think they shot it over a couple of years as everyone had free time here and there. I'd bet most, if not all the actors, took the minimum salary the union would allow them to take on a studio movie. If it's not a studio movie, they probably did it for next to nothing. Like you said, "hey, so and so's doing it, so I will too."