A good first guestimate is usually approx. $1.5 to £1. It kind of works, and is much easier to convert in your head!
Obviously if you want to know more precisely use a currency calc.
The thing that really scuppers me in the States is sales tax. Because the 17.5% in the UK is added on before buying, I'm not used to having to add the 6% (or whatever) in the States!
Ellie