Norrath GDP questioned

Journalist Mark Wallace questions Ted Castronova's Everquest GDP calculation. According to Wallace, the GDP should have been around 400 US dollars rather than 2200 (at the time Castronova's article was published).
rI have no idea how to calculate the GDP (well, I do have an idea, but it is not my field of expertise) but I think the question is worth asking. Let me know if you get any updates on these claims.