NB These are from memory! I'm sure someone else will come along with accurate figures.
Anything valued at less than �18, they don't bother with.
Above that, your stuff is subject to import duties, the level of which vary from 0% to 40-ish%, typically around the 5-10% level.
Then you get VAT added onto the post-import-duty price. And (though I think this bit is a bit dubious) they VAT you on the carriage charges. (As these were paid (and taxed) in the country of origin, for a service in the country of origin, I don't think this is right - but what can you do?!)
Finally, the company handling all this for you EG FedEx, Royal Mail, will charge you for their services.
As a very rough rule of thumb, I always add 25% to what I paid to cover these charges.
However, if they are busy at the importation port, and you're lucky, you might get it through absolutely free. They don't check & charge every package.