I recently saw somebody ask, "Who makes more--the top-paid novelists, or the top-paid screenwriters?" I thought that was an interesting question, so I did a little number crunching. Here's what I found.
But first, a warning: what follows is based on anecdotal data, rather than statistical. And the anecdotal data tends to consist of media estimates of private wealth. If those estimates are off, my conclusions will be way off base.
Now, with that out of the way:
According to Forbes, JK Rowling is a billionaire; if that is true, it makes her the first person to become a billionaire just by writing novels. (She has said she is actually not worth a billion dollars, but I think we can assume the Forbes estimate is roughly in the ballpark.)
Clearly, there is nobody who has made anywhere near a billion dollars just by writing screenplays. So by that measure, novelists win.
What about if you include writer/producers, or writer/directors? Well, George Lucas is worth an estimated $3 billion. So by that measure, Hollywood wins.
I think it's safe to say that JK Rowling, as the best-paid novelist in all of human history, is an outlier. What about a relatively low-paid writer like, say, Dan Brown or Steven King? Well, Steven King's net worth is estimated to be "above $200 million." I suspect there are no screenwriters who have net worths that high from just screenwriting_--but if again, if you include writer/producers and writer/directors, then suddenly Steven King doesn't seem so wealthy.
I can't find any estimates of Dan Brown's net worth online, but Forbes estimates that he earned about $88 million last year. Given that we're dealing with very rough estimates, that's in the same ballpark as the $70 million that "Law & Order" kingpin Dick Wolff took home, so on that measure, novelists and screenwriters are about tied.
So that's the megastars and the mere superstars. What about the average joes?
When the WGA releases its annual report, it usually turns out that only about 50% of WGA members had any guild-covered employment in a given year. Among those who DID work, the mean income is usually about $90,000. (I'm quoting from memory, so these figures are approximate.) So, if you are unemployed half the time and earning $90k the other half, your average annual income is $45,000.
On the prose side, $10,000 is a pretty standard advance payment for a novel. So if you wanted to earn $45,000 in one year, your publisher would have to sell out your initial print run, then sell out two-and-a-half further print runs of equal size. I suspect most novelists do not manage to do this, so it looks like screenwriters win again.
However... and now I'm venturing into a realm of wild speculation, so take this with a grain of salt... I suspect that any given novel has a better chance of selling than any given screenplay. The minimum cost to buy a novel is less than the minimum cost to buy a script. And the average cost to actually publish a novel is less than the average cost of actually making a movie. And screenplays have fewer words, so I suspect more screenplays get written than novels... which means there are probably more unsold screenplays than unsold novels...
So if you write a novel, you are more likely to get paid something for it than if you write a screenplay.