As you say you dont know much about them, jump on to the main brand websites and have a look. Most of them have RRP on the site, so that will give you a rough idea on what you will get.
Thennn, once you have picked which one you want, slip on a nice top, wander down to JB HIFI, and ask them for a deal (ie say what you want, and how much you are going to pay for it). No doubt they will be looking at your boobs not the cash your handing over, so you'll walk away with a bargin!