Writer Craig Maizin (co-writer of Scary Movie 3) gives us an idea on how much screenwriters (should) get paid in Hollywood: The Economics of Screenwriting
There are a few basic ways to get paid as a screenwriter. You can option literary material, you can sell literary material, you can pitch an idea, or you can be hired on an assignment.
Options don’t technically fall under the WGA’s jurisdiction. Options are just rental agreements. The optioner pays the optionee a fee that grants the option the exclusive right to “set up” the project at a studio (typically as a producer). The writer will then sell the literary material to the studio.
If you sell a script, the studio has to pay you scale. “Scale” is just a term for the basic minimum amount. Right now, if you sell an original screenplay for a “big budget” film (a film that costs more than $5,000,000), scale is roughly $77,000 (including one additional rewrite step). You can learn about all of the various minimums here.