(This is a round-up of miscellaneous useful information rather than a structured examination of a specific widespread misconception about publishing.)
How much are SF/F novelists paid?
A few years ago, Tobias Buckell got curious about this question and ran an anonymized survey. In 2005, he re-ran it, and his full results are here, with input from 108 authors. (Note that his figures refer to the US market.)
If you want a full run-down, I strongly recommend reading the whole thing, with discussion, but in a nutshell: the median advance for a first SF/F novel is $5000. For authors going through an agent it was $6000; for unagented novels it was $3500. (As I noted in an earlier CMAP posting, an agent is on commission and takes 15% of the author's cut. And they're cheap at the price, going by this finding!)
For established authors, the median advance for an SF novel is $12,500, and for a fantasy novel it's $15,000. Agented books average $12,500; unagented average $7,250. The range of advances is much wider in fantasy; the survey logged advances of $0 to $40,000 in SF, and $1000 to $600,000 in fantasy (the latter being a one-off that should probably be excluded from analysis of the results).
Note that, as I explained earlier, the US book advance is not the whole story; royalties and foreign rights sales may add considerably to the pie — or not, if the author or their agent don't exploit them properly.
How do SF/F novelists break into the business?
Fantasy author Jim C. Hines got interested in this question and ran a survey of his own, just last month. The results are fresh in, with input from 247 writers, and Jim discusses his findings in two essays.
In the first he examines the received wisdom (at least in the SF/F field) that you break in by writing and selling short stories, polishing your skills, and then work your way up to novels; and he looks at whether self-publishing your first novel is a route to success. The full story is here. (Attention conservation form: writing and selling short stories can help some writers, but is by no means a universal route to success; and self-publishing your first novel is almost certainly useless.)
In the second essay, Jim examines a couple of other myths; the myth of the overnight success, and the widespread belief that you've got to know somebody in publishing. I'm not going to spoil his masterful examination of these issues: read the whole analysis here.
Note that if you see a conflict between my advice and the advice in these surveys, go with the survey. My experience and career path are very atypical in some respects (although utterly vanilla in others — the age at sale of first novel, for example, is 36).
Finally, if you're interested in writing and publishing (especially SF/F fiction), there are a bundle of resources online. If you're writing purely for your own enjoyment, you don't need to bother with them — but the moment you put your writing in front of someone else's eyes, you're dealing with other folks' expectations, and you'll find that it helps to know the basics of how to prepare manuscripts, how to participate in a workshop with other writers to constructively criticise each others work, and how to approach a publisher. SFWA, the Science Fiction Writers of America, run an online information centre with essays these and other relevant topics: highly recommended as a starting place.