Daniel,
I did buy my Nauticam GH5 housing at Backscatter in the US back then, as it was not becoming available in Europe in time for an upcoming dive trip. I searched for the customs bill but could not trace it. However, what I do recall is that all-in-all I ended up paying around 10% more than what I would have payed buying in Europe.
Therefore, unless there is a special situation (item not (yet) avail in Europe or super deal for a used housing etc...) I would advise you to buy in the EU. Also these custom rates are sometimes unpredictable depending into which category the officer places it. Also, unless your sender uses a high quality carrier (UPS the best for US-shipments), sometimes stuff gets stuck for a week or two in customs. Not always the best experience!
onderwaterhuis.nl in Holland is my preferred and highly trusted supplier in Europe for UW camera stuff these days.
Burkhard