Gamasutra has a very interesting report on a recent study of the virtual item economy. More men do it than women, but women spend more money on average. Medians are much closer, indicating that much of the difference is in the upper tail: There must be some women who really buy a lot of virtual items.
Among those who buy virtual items, the average spending amounts are in the $20 - $50 annual range. A very high percentage of the sample (drawn apparently from people who are in games and on social media sites) engages in some sort of real-for-virtual transaction. It seems to be mainstream, normal behavior within this group, and this group, as we know, is getting bigger and bigger.
Thanks to Ed Gentry for the tip.