We've updated the XYZ Collectibles website so it now has a "Price History" feature, which you can use to find out how much an individual issue of National Geographic is likely to cost (or what it might sell for). The page is
Enter National Geographic in the collection name field and press "Go" and you'll get a list of recent sale values. Anyone can add more price data, but you have to be logged in -- guest users can't add sale prices.
I took about a hundred recent eBay sales and used them to get the list started. I'll add more periodically, but it would be great if others here could also add sales data from issues they've bought or sold -- the more data we add, the more reliable the information will be, and that helps everyone.