This actually surprises me. With someone as thoroughly read and prolific as Stephenson I would have expected a much larger financial reward. Are the economics of publishing really that bad or is he just being modest?
If he gets a dollar or two per copy (which I think is roughly correct, based on my very limited understanding of the industry) then that’s $3-6 million. He had been writing for around 30 years at that point, so that’s in the neighborhood of $100,000-200,000/year.
Excluding outliers like Harry Potter
However, I think the custom is to buy a time-limited option first for 10% of that sum, say, and only pay the full sum if the film really gets made.
One can always ask for a percentage of the gross, although it's rare that something like that is agreed to by everyone concerned. The producers of the original "Lord of the Rings" films felt for some reason kind of desperate at one point and tried to get Sean Connery to play Gandalf. They offered him a percentage of the gross to appear in the film. I did the math on it one time and if I remember correctly it would have paid over half a billion dollars...
[1] Not only with other people who are writers, but with the attention of potential customers (games, movies, surfing the web, etc).
The Society of Authors (https://societyofauthors.org/) do some excellent work on this front. Basically, to make money writing you have to either be a _machine_ and put out a lot of content, or get lucky and build a fan base that is willing to wait for infrequent releases but buy all the copies.
I know Sanderson employs at least one assistant.
Stephenson has written apparently 16 novels in 35 years. [1]
Sanderson, on the other hand, has written around 30 novels in 14 years, depending on how you want to count novellas and kids' books. [2]
Sanderson is also much more active in keeping up PR about himself and what he's doing -- I think that fan interaction, both online and offline, is as much a part of his popularity as his actual writing. He's fantastic in the respect and I'm sure it reflects in his sales numbers... and probably helps with all of his discussions in getting his stuff into TV/films/games, and a lot of that stuff is (as far as I know from what he's allowed to discuss) in progress.
So I'd actually expect to see Sanderson making much more than Stephenson and it definitely makes sense to me that Sanderson would have an assistant or two given all of his engagements and franchises.
[1] https://en.wikipedia.org/wiki/Neal_Stephenson#Bibliography
[2] https://en.wikipedia.org/wiki/Brandon_Sanderson_bibliography
While Sanderson has several film projects in the works, none of them seem far enough along to have paid off yet financially. Although I guess it depends on how the contracts were written.
The really interesting part is that the article MikeAsh linked also lists Sanderson (as of 2013) with 2.5 million sales of his solo books, meaning that he sold at least another 4.5 million books in various languages in two years, basically eclipsing Stephenson's lifetime number.
Given the difference in their output and the times that the various books were available for sale, this really cements the difference between them sales-wise for me. My gut feeling here is that a lot of this is due to not only his writing and stories, but also to his constant enthusiastic fan engagement.
[1] https://news.ycombinator.com/item?id=20259853
[2] http://awfulagent.com/brandon-sanderson-sells-over-7-million...