AVGAS price
The European cost of fuel can be tempered by going in and out of the UK, The Republic of Ireland and the Channel islands with the drawback of duty on the fuel.
The Europe vs USA cost issue is not as clear as in might seem and requires a lot of looking at not just on a cost An hour building basis but also on the basis of the extra training that you are likely to require when you return to Europe for your ATPL course as undoubtedly you will need some extra navigation training to pass the skills test in a European environment.