There are many large companies who have regional i.e. country specific websites. The product lists and specs vary from region to region as do marketing needs and methods. So although any site can be reached from anywhere usually, the customers go to the site that is for their country.

Never ever be afraid to quote big numbers for your images, it's only scary the first couple of times. Be aware that even now, sales into the many thousands of dollars per image occur often.

How can a website not be "worldwide" geographic distribution? 

Corbis has a nice pricing utility, and some nice descriptions about the different choices.  for example, it specifically states "if you are licensing your image for a web-site usage, select the geographic distribution option that best represents the location of the site's primary audience" ( reference here)

Zeus brings up a great point about regional sites, and if they only intend on using the image for one of the regional sites, that would also change the geographic distribution.

When coming up with pricing, we try to give the client a break when they're regional but worldwide.  e.g. an instructor in Florida that potentially draws students worldwide, their primary student base is going to be local, and we consider the distribution to be that.

Now, things do change if you're talking exclusive vs. non-exclusive; and that's where you see significant need to increase prices based on geographic distribution.  If you are going to give the website exclusivity, then they need to pay for the coverage they wish to have.  If they want worldwide exclusivity within the education industry; then they definitely aren't going to be charged for just a US distribution; since it then limits us for sales to that industry in that region and regions that include the exclusivity region.