As an example of buying from the US, I just bought some machine heads from Hipshot in the US.
The parts cost $174.24;
Shipping (USPS with $200 insurance) cost $61.71;
Customs Duty - £3.32;
Import Vat - £33.87;
Clearance Fee (zero Vat rated) - £12.00.
Total Cost - £244.59
Original Cost of parts - £144.30
The package also took 2 months to arrive, due to various cöckups on the way.