As I started with, you want to create a win/win. I'm assuming you're not being paid because it's a low budget, non-union production company. They want to produce your script because they believe in it. Therefore, talk to a literary agent, manager, or entertainment attorney, who can give you some advice on signing a contract to receive money should it sell. You may also be able to add a few more things to the contract in the event that the movie "takes off."
The reason I advise you to speak to a professional, is because, without contract experience, you could end up losing a lot of money. Number people can be very tricky, and it may not even be the production company you're dealing with now, but the one that they sell to.
Approach it like you would a pre-nup, with love, trust, and "a teensy need for insurance." You appreciate the opportunity, you believe in the producers, and therefore, you want to know that they are just as invested in your success in the end, since you're willing to sacrifice in the beginning. If a contract is going to be a deal breaker, you have to decide if getting your first screenplay produced is worth it. Either way, congratulations on getting this far.
For more tips and articles by top entertainment industry career coach, the Greenlight Coach, visit www.TheGreenlightCoachBlog.com