Hi there,
Not sure at all of what you mean...
If you are in the UK, using an English section of the usual big online comparison sites, you'll see the price listed and it'll also show you the taxes amount etc.
If you then go to a USA section of a similar website you'll find prices listed there. You may have to click forward to get the amount of taxes to be added. You can go to a US website, get quoted for a flight from and to wherever you want.
The usual tricks apply, keep checking different sites, and different times of the day to see how prices and availability changes. If a deal comes up at a price that's good, its your risk to either accept and book there or to try and see if a better deal comes up later.
No matter which country you are in, you'll pay the taxes accordingly, even if you end up booking from the UK for a flight that originates and returns to an airport in the US and vice versa.
Cheers, DtM ! West London & Slough UK!