Approximately 60% of gamers say they have bought some sort of virtual good within a game

"Free-to-play games, where you can play a game for free but are then encouraged to buy virtual goods such as weapons, have taken off. Even though this business model is like a return to the expensive old days when gamers paid a quarter for minutes of play, hardcore gamers are embracing virtual goods.
About 88 percent of 4,816 gamers surveyed said they had purchased some form of digital content, including music, movies and games. About 60 percent of respondents said they had purchased a virtual good inside a game (where the virtual good was not a full game), according to a new report by market researcher DFC Intelligence and virtual goods platform company Live Gamer.
Many of the virtual goods games fall into the category of MMO Lite, or relatively casual massively multiplayer online games. These games don’t have the high monthly fees of World of Warcraft, which charges $14.95 a month. DFC Intelligence forecasts that the North American and European MMO Lite market will grow from $800 million in 2009 to $3 billion by 2015.
Gamers in both Europe and North America are now comfortable with buying digital content, the report said. Buying virtual characters, weapons, decorations or other items is now common in games ranging from Zynga’s FarmVille on Facebook to Nexon’s Combat Arms. These sorts of online games have hundreds of millions of users around the world."
Source: Research from DFC Intelligence and virtual goods platform company Live Gamer, reported by VentureBeat, 29th March 2010