A 20% price increase before VAT is still expensive, especially because American wages are much higher (and often come with a higher cost of living, yes, but that still makes it easier to save up for a tablet).
Another advantage is that most of Europe is more densely populated. Google sells their 500 dollar tablets for the same price everywhere within the states, whether you live in the center of New York or the middle of the Nevada desert. They already need to cover the costs for this, so I don't think transportation costs cover the price difference.
Perhaps EU labour laws protect warehouse workers better, that's a possible reason for such a steep price hike, but I don't think warehouse workers earn that much more here.