I tried searching for the net worth of various countries, but couldn't find much online. http://mjperry.blogspot.com/2008/09/us-household-net-worth-up-by-43-since.html fed says US household net worth at 56T$, but that is not the net worth of US, you have companies and the government (specially the military) but I'm sure there are alot of countries whose net worth is less than 2T$ do you know any links to such data?
If US household is worth 56 trillion dollars and it has 13.8 trillion GDP then New Zealand's household is worth about haft trillion dollars. Add on national lands other government properties I guess you can bearly squeak by.
I would just take the average price of a piece of land per acre in that country and multiply by the total acres of said country and that should be your buying price.