I've done some odd things to answer AB questions but today's the first day that I've ever put my jeans onto the kitchen scales!
The weight was about 1 lb 3 oz. I've used imperial units because that's what the US postal website uses:
http://ircalc.usps.gov/
The range of possible prices is quite wide, depending upon the service used. However, airmail parcel post is quoted at $21.10 (about a tenner).
Remember that it's not the postage which is the big expense when importing goods from outside of the EU. It's the duty, VAT and customs examination fee which can really add up.
Let's suppose that the dress is advertised for sale at the UK equivalent of �50. The postage will be around a tenner. (that's assuming that the vendor doesn't charge for 'packaging'). So you pay �60.
When the dress arrives, HMRC will calculate the duty to be paid. This is normally 12% on most clothing items but, because the USA is currently in breach of World Trade Organisation rules on subsidies to US cotton farmers, the UK government is adding an extra 15% on as a form of trading sanction. So that means that you'll have to pay 27% of �50 in duty (=�13.50).
HMRC then calculate the VAT to be paid. You pay VAT (at 17.5%) on the dress, the postage and on the duty. (Yes, you have to pay tax on a tax!). So, the VAT is 17.5% of �73.50. (= �12.86).
Then, to add insult to injury, you have to pay a flat-rate fee (�8) for the privilege of having HMRC examine your goods and charge you money!
So, when the postman calls, he'll demand payment of �34.36 before handing over your dress. That means that your �50 dress will have cost you �94.36.
Chris