I wrote up this response:
I've been submitting short stories to magazines; if and when one of those sells, I might get a couple of hundred dollars (depending on the publication). The short fiction is mostly for practice, and to accumulate professional credits.
After I finish this month's rewrites/additions to the novel, I'll start querying it; if it sells, a typical first-novel advance is about $5,000. (Last year at VP, I talked to one of the staff who had signed a 3-book deal for a total advance of $60,000. That's insanely great for a new novelist. Her agent had managed to get two publishers into a bidding war.)
More on first novel advances:
I also have a couple of screenplays in the trunk. I have no idea what the going rate for those is, but my impression is it's highly variable depending on a variety of factors (no pun intended). I'd have to do quite a bit of work to get either of them into fighting shape.
Realistically, I probably won't ever make a living from writing alone; most fiction writers don't. I expect to get back into web consulting in a few years--I've already started attending the various open-source events here in Portland--or doing other types of freelance work.
Those are the facts. I'm luckier than most to be able to take a few years off to chase this dream--I've already won the lottery once, figuratively speaking. I have no illusions about my chances of becoming a bestselling millionaire novelist (to wit: vanishingly slim), but I believe I have a good chance at getting published, sooner or later. I'm working on the "sooner" part.