Someone on one of my MLs has just posted this link. It relates to romance fic advances, but as far as I can tell from past experience, the figures are accurate with regard to some of the publishers who also do genre:

http://www.brendahiatt.com/id2.html

Interesting to see. It's possibly worth noting that this is only for the US - UK advances tend to be higher on average.

Other people's input is welcome on the comments list, BTW!