If the import is less than £18 the chances are you won't get hit for VAT (which is what you usually have to pay on import rather than import duty). Technically, if the item isn't a gift, duty is payable on any amount, but often they seem to get through.
Above £18, you may well have to pay VAT at 17.5% on the whole amount, including the shipping. Whoever deals with the import, Parcel Farce etc., will also want a good slice of moolah for doing all the paperwork for the customs and excise, and this handling charge is often about £13.
I bought my Tippy A5 from a guy in the States off ebay (as they can sell markers on it there) and got a good deal even allowing for VAT and handling charge.
If the price difference between USA and UK is smallish, I wouldn't bother, at least not until the pound is stronger again (bought my A5 at about 2:1).
Phil