This is so old that it’s almost embarrassing to post it! But I didn’t have a blog back in 2005.
Author Tobias Buckell collected and analyzed a bunch of data (from a survey he took of published authors), to determine what a typical science fiction or fantasy writer is paid for a novel. There’s lots of interesting and useful information here, breaking down the data to look at the typical advance for a first novel, for a subsequent novel, for fantasy vs. science fiction, for a novel sold with the help of an agent vs. a novel you sold all by yourself (the data strongly suggest that you are likely to get paid more if you have an agent, even subtracting their percentage).
I think the vast majority of my writer friends have already seen this post, but I thought the one or two who might not have would find it useful. Also, it will hopefully convince all my non-writer friends and family that I’m unlikely to ever get rich from writing.
Median advance for a first novel: $5000
Median advance for a subsequent novel: $12,500
If you’re doing it for the money, Starbucks is a better bet. Better benefits, too!