However, I think these posts might be a bit too dire. Way back when I got my first book deal in 1997, it was for $12,000 for the first book and $15,000 for the second, so far more than the $5,000 or $6,000 that many of these examples start with. Maybe mysteries pay a little better than the horror or science fiction they were talking about in the linked articles. And as your career moves along, hopefully your career is building and your backlist is still selling.
Plus there are other ways you can make money as a writer, although none of them are sure things. Audio books, foreign rights, book clubs, movie options. If you write books for kids, especially younger kids, you can get paid to do school visits. (My first five books were for adults, so it was a real shock to learn this.) And books for kids tend to stay in print for a long time. My first YA came out in 2006 and is still, as of today, in print. Anything that has gone out-of-print I have put back up as ebooks. A few times when things were lean I taught classes at my local bookstore.
Nothing is guaranteed, of course, but for past couple of years I have made well more than I did when I worked in corporate communications for a big health care company. I will say that my husband has helped tremendously by having a job with health insurance and regular bi-weekly paychecks. I've also learned that money you don't expect to come in does and money you did expect to come in ends up being delayed for months, but somehow it all balances out.
The one thing I don't like is that it's very hard to crystal ball it, hard to look more than a year into the future. I'm getting better at living with that uncertainty. If you are self-employed, I highly recommend The Money Book for Freelancers, Part-Timers, and the Self-Employed: The Only Personal Finance System for People with Not-So-Regular Jobs. You need to get good at paying your retirement, your taxes, and your emergency fund up front. This book has helped me do that.
Author Jim Hines looked at his finances for 2012 here. He also has a full-time job in addition to his writing.
Anyway, if you are thinking of quitting your day job, don't despair. I believe it is possible. You may find yourself working like a dog, but you'll also be making up stories, your commute will be from your bed to the computer, and you will be your own boss. Even if I was making less (and who knows, I might soon be), I would still say it was worth it.