It does depend on the starting price.
For a wireless router, as Scott has mentioned, the price differential from U.S. to U.K. is not that great; but for items that are early on the market it might be.
For example, you can buy at 802.11g router in the U.S. for approx. $100 (about £65 at the current rate). In the UK you will pay about £90 net. However, the shipment cost by Fedex or DHL would be abour £35 since the post office service on both sides of the Atlantic is incompetent and not to be trusted.
On a single item purchase like this, it isn't therefore really worth it. However, if the value of the order is $200 or more, then it is interesting to purchase from the U.S., as long as the item has a universal power supply.
There is no duty on this type of equipment or software, only VAT, which of course is also payable for local purchases.
Generally commodity items such as motherboards, memory and CPUs are not worth importing - other items may well be.
The other option is to take the opportunity to have a cheap holiday in the U.S. I believe that the duty and VAT free allowance on goods from the U.S. is now £145. In that scenario, you have no shipping cost either, so you are a fair way towards the cost of the ticket..... One issue is that if you go to a retail store, you will generally pay sales tax, whereas internet orders do not carry this for international shipments. However you can choose to visit a state such as Delaware or Montana which do not have sales tax.
.andy
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