WIFI in USA and UK

1- Why they are so cheap in USA and expensive in UK?
Turn the question around, WHY is EVERYTHING more expensive in UK ? Bare bones broadband is 4 Euros (yes, four, or actually 3.99 !) p/m in Germany.
2- Can I buy a WIFI product in USA and us it in UK? (Power supply, standard, protocol, etc)
Yes, but apart from power supply problem, some may be illegal here as they have up to 200mW output, UK & Euro limit is 100mW. Also US uses PPPoE, so their routers often don't have choice, but BT now handles both UK std. PPPoA and PPPoE, so it's no problem now.
3- Can I buy from USA and ask the seller to send them to UK? Should I pay any tax? If yes how much?
You can ask, they can refuse and / or charge a lot. Yes, you should pay import duty (10% I think) and VAT. Regards, Martin

Is it? Do the maths.
Its being sold at a loss, to attract clients to expensive addon services ? Its being run on preexisting hardware already paid for out of some other budget?
No.
And of course you have no warranty. Or support.
And insurance, and shipping charges, and airport handling charges.
I did this once. Something billed at around 25 quid cost me over fifty by the time I actually got it.

scale? import duty? VAT? depends some manufacturers such as cisco have different models (although i thin you can change the country setting in the s/w) Netgear 802.11b/g seem to ship the same tin worldwide - part of the install asks you to choose the country so the box can configure the number of channels etc.
But a US box will come with the wrong mains power supply - wrong plug and voltage.
802.11a wasnt covered in the UK regulations initially - not sure where that is up to. you can - and customs will want their cut as well as the VAT..... > > Best regards