Shopping - Brand named clothing - Genuine products - Cheaper prices

Hi all,
I am looking to buy Brand named clothing such as Ralph Lauren, Abercrombie & Fitch, Tommy Hilfiger, Calvin Klein, etc. etc. I want to top up my wardrobe since it is quite empty after arriving here recently.
Does anyone know where I can buy genuine Brand named clothes at low prices? I normally do my shopping in the States since Ireland is so expensive for these brands in comparison. The majority of these brands are produced here in Vietnam so therefore I expect them to be a fraction of the price....Right??
A Ralph Lauren polo shirt for example in the States costs me $45 - $90 approx. What should I expect to pay for them here?
I understand that there is a lot of knock off clothes here but its obvious they are fake & the quality is awful.
If you could advise me as to where is best for buying such clothes & ballpark prices it would be much appreciated.
Many thanks guys,
Johnny