SFCrowsnest.com, "Europe's most popular science fiction & fantasy site" (??), recently surveyed SF and fantasy writers on their advances & incomes. Among the results:

• The range in Fantasy first novel advances is from $0 to $40,000.
  
• The average first novel advance is $6966 for Fantasy.
  
• The median first novel advance is $5000 for Fantasy.

• The range in Science Fiction first novel advances is from $0 to $20,000.
  
• The average first novel advance is $6555 for SF.
  
• The median first novel advance is $5000 for SF.

Other stats take a look at the difference between advances received by agented vs. unagented authors (please don't roll your eyes like that...), advances for first novels vs. subsequent novels, and advances for hardcover vs. trade paperback/mass market releases (...your eyes might get stuck).

Correction: Beatrice informs us that SF writer Tobias S. Buckell originally posted the survey's results on his weblog. SFCrowsnest.com simply reprinted the entry.