PlaySpan and VGMarket surveyed just over 2,200 people between the ages of 13 and 64 regarding their purchases of virtual goods and currency. Women are the big spenders here, particularly when it comes to in-game money. They spend an average of $50 per year as opposed to the $25 a year that men spend.
The gap is a bit smaller but still significant when it comes to actual in-game items. Women spend an average of $55 a year and men an average of $30.
The survey covered a few other similar results, including combined spending and a different result when overall median spending was compared. The full survey is an interesting read, particularly considering the increasing popularity of microtransactions in the MMO world.