Unfortunately, that number alone doesn't tell us much, except that there's a lot of gold moving around in Northrend (it would be interesting to compare this to, say, Azeroth or Outland's equivalent, though the more useful numbers would probably be Outland before the new expansion hit, when everyone was still farming and selling items from there). And it will be interesting to see this tracked in the future: the real GDP is usually used as an indicator of both standard of living and a country's economic health, and while there are drawbacks to using that number to gauge both of those qualities, it's probably fair to say the economy in Northrend is booming. Maybe tracking this in the future will let us see how new content patches or item or even class updates can affect what the economy does there.
Very interesting. EVE Online's creators, CCP, have actually hired an economist to help run their ingame economy, and while WoW's isn't generally seen as quite that complicated, there are still plenty of big numbers to play around with..