Though you will see advice on the web that says something along the lines of "Think of 1 USD = 1 UKP", I don't think that's right. As I write, the exchange rate is 1 USD = 0.5383 UKP, 1 UKP = 1.8576 USD. In these terms, a dollar is worth a smidge over 50p, so think of the seven-sided 50 pence piece as the dollar coin.
But it's probably more complicated than that. Value Added Tax (currently 17.5%) is always included in the stated price here, whereas in the States I understand the tax is not.
There are excise taxes too, which are also included in the price - for example on fuel, booze, tobacco etc - these change with each Government Budget according to how much the Government wants to encourage/discourage us to buy things (among other arcane policy decisions involved).
Also your perception of what is good value will depend upon whether your income is paid in dollars or pounds. My colleague Molly is going to the USA on holiday this summer and is rubbing her hands at how cheap things are going to be over there; someone coming in the opposite direction is going to be wringing their hands - the dollar in their pocket is not going to get them as much.
Other things that affect the cost of things is how many or much of them there is to go round. A crude example: there are 622 people per square mile in the UK, but only 80 per square mile in the States - so there is less land to go round here, therefore real property is more pricey. Also things are manufactured in greater variety and in greater numbers in the States, therefore you'd expect them to be cheaper there. Then again, some things are manufactured in the US that are scarcely manufactured here - eg., DVDs and software. So the producers can to some extent fix the price to their advantage, hence the 1 USD to 1 UKP equivalence in those commodities.
I've just asked Molly if a blouse would be good value for ten quid and she says, yes it would be!
I'm not a economic/fiscal/financial expert (bloody hell! I can scarcely balance my own household budget!) so the above stuff is probably not very well informed - it's just my way of working it out.