Above you see a (partial) screenshot for the first result if you search for harry potter and have selected the Netherlands as your country. For every result CheapRiver.com shows a table with the prices of the book in United States, United Kingdom, Germany, France and Canada.
The lowest prices are indicated with a green background. Here you see that the cheapest is to buy the Harry Potter Box set in the US to have it delivered to the Netherlands it only cost (on Dec 13 2009) 37.26€ while in the UK it'll cost you 51.03€, Germany 58.99€, France 63.77€ and in Canada this version of the Harry Potter Box is not available. So you see that the cheapest option is more than 40% cheaper than the most expensive one.