tax is different from state to state, when I was in Seattle it was around 10% could be more or less, google will probably tell you.
I nearly bought one when I was there, but when you add the cost of a power adapter (£26) and you can't just take it back should it go wrong, didn't seem worth the £40 saving, yes I know it has a warranty, far easier to take it to the shop you purchased it from.
If you are looking at a 3G sim card one, they are locked to the provider, AT&T or Verizon, so another reason not to buy it.
some peripherals were cheaper, cases etc.