Depends on the budget of the movie. If it really is big budget, maybe you could squeeze $50,000 out of them. I guess it also depends on what they are asking from you, will you be explaining Einstein's theory of relativity in detail or will you be saying things like "hey there's no gravity in space."
I would look up the average yearly salary of a college science prof. and divide by 6. That should give you a good starting point.