Most likely they'll want to take an option, which is temporary ownership of the movie rights. An option enables a would-be producer to show your work around to investors without worrying that one of them will get in touch with you and try to buy the rights out from under him. As long as he has an option, he "owns" the movie rights.
Options are usually for a year or so. When it's about to expire, the option-holder can renew by paying you more money.
When an option is negotiated, the final selling price of the full movie rights has to be established. Otherwise, the producer would be showing the work to investors without being able to tell them how much the work will cost.
Let's say I offer you $15,000 for a one-year option, against a full purchase price of $100,000. The first option price is applied to the full price if the deal goes through, so if I get all my investors, I owe you $85,000 for the full rights. But if I don't get all my investors together and still want to push the project, I can offer you another $10,000 (or whatever) for another six months (say), but that 10,000 is not applied to the purchase price. So if I end up buying an option for 15K, and three more for 10K each, and get the green light, I still have to pay you 85K for the full rights.
And of course, your agent gets her percentage of all this...
It's true that most optioned works don't get made into movies, but as long as you keep selling options, why worry?
I have mutual friend of Walter Mosley, who wrote Devil in a Blue Dress, and he tells me Mosley kept getting renewed options on it every six months for something like three years. By the time they finally made the movie, he kind of missed the steady deliveries of money. Of course, your mileage may vary...
The price of the option and the final selling price are whatever traffic will allow. Could be huge, could be peanuts. Depends on how bad they want it, and how willing you are to refuse a deal you don't like.