Northrend's GDP would be the value of items produced, plus gold gained from questing and drops. Exports don't exist, but as for Government Spending the closest thing to taxes would be revenue spent on the mail system, repairs and vendor goods, which would count. Investment costs would be money spent on NPC crafting recipes.

Rather than attempt to attribute the traditional aspects of GDP, realise what the GDP formula is meant to examine: The production. Basically, all gold spent in Northrend is a way to track player production. Items are the bulk of this, although IIRC you'd have to discount the prices of intermediate goods from GDP, so crap greens bought to DE would be counted twice. I could be mistaken, it's been a while since my macroecon class and micro is a good bit more fascinating.

I think what's not being observed is the RMT implications of any GDP calculation. It's been a while since I've seen goldspam on /2, but IIRC 1000G was being advertised at roughly 20 dollars. Applying that to the item-only total of 719,000,000 gold gives a figure of about 1.4 million dollars. Now due to the black market nature of WoW RMT, gold isn't as liquid or tradable as real world currencies, thus the exchange figure is slightly high. However, considering that the item-only part of this is certainly an underestimate of the true GDP of Northrend, (and doesn't factor in Vanilla/BC apparently, which is still a massive part of the economy) the actual real world production value of WoW in USD is fairly eye-popping.