At $18 per person per month, that's an aggregate in-world trade volume of $2.2 billion annually. According to UN data, that volume of trade would make virtual worlds the 144th largest economy in terms of GNP, right up there with Malta and Albania. On the other hand, this number is such an extreme telescoping of assumptions as to have little merit in an absolute quantitative sense. We only have the velocity number - the keystone of all this - because Philip Rosedale of Second Life was kind enough to let me see it. It's frustrating that we don't have better data, and that we're forced to put together this crazy-quilt patchwork of assumptions to get even the most basic feel for what's going on. (If you're the proprietor of a big game and can see how some of these data, if released, could benefit the entire industry, get in touch.)