There are an estimated 400,000 'gold farmers', mainly based in China

"In basic terms, gold-farming [the production of virtual goods and services for players of online games] is a sizeable phenomenon. The rather wobbly-legged best guesses for 2008 are that 400,000 gold farmers earning an average US$145 per month produced a global market worth US$500m; but we could easily more than double the latter to over US$1bn. There are probably 5-10m consumers of gold farming services. The main uncertainty of estimation relates to the gold-farming market in East Asia, which appears much larger than that in the US/EU. That uncertainty in part arises because gold farming operates at four levels – local, national, regional and global. We should encompass all four but, to date, the focus has been almost entirely on the global trade."
Source: Page 66 of Current Analysis and Future Research Agenda on "Gold Farming":
Real-World Production in Developing Countries for the Virtual Economies of Online Games, by Richard Heeks, Development Informatics Group, IDPM, SED, University of Manchester, UK
2008, citing Wang, R. (2008) The Truth Behind Gold Farmers, WoWMine, Wilmington, DE
This excellent paper will tell you all you need to know about gold farming!