Not a heckuva lot (surprise, surprise), according to Tobias S. Buckell's informal survey of 74 writers:
The range was from $0-$40,000 for an advance on a first novel.
The average was $6363.
The median advance is $5000. The median figure is a better indicator of what most people consider 'average.'
Payments tend to go up, of course, the more you publish, and if you can get agented, but most professional science fiction writers main source of income isn't from sf, in my experience, or even from writing. Most have a day job, which makes their ability to finish anything even more impressive. Most of the successful writers I've met are driven to write. The money has had little to do with it.
My advance for my only piece of professional sf/fantasy was $1,500 for "Once in a Lullaby", a novelette published in Bantam's first "Full Spectrum" anthology, and apparently not a bad payment, considering that sale was almost 20 years ago. Although I still dabble in fiction writing, and suspect there will be a day when I return my focus to it, the effort/reward ratio as shown above forced me away into other things that paid better and more consistently; multimedia development for about 10 years, now back to freelance business writing.
There are times I regret that choice... but not all that often. And I still have time.