It mostly depends on the weight of your letter so its hard to round an exact number. You could try the calculator here http://ircalc.usps.com/?country=10150 but I wouldn't go with less than a few dollars or more.

Also, I believe the usps eBay store ships worldwide if you want to try that for stamps instead. :)

Tried loooking on the US postal website and couldnt working it out, if i send a large letter to someone in USA to return to me in UK how much $$$ should i include to cover their postage?

or does anyone know where i can but usa stamps online if not?

cheers