The average retail cost at a place such as Sunglass Hut appears to range between $140 and $200, depending on the style. Cost at other retailers may vary.
Dolce and Gabanna sunglasses are about 300 dollars or so. You can buy them from cheaper on ebay as well and get them from a lot cheaper than the actual store.
It depends which style of Dolce & Gabbana sunglasses you buy, but you can probably get a pair of them for about $150. These are the cheaper end and can go up much higher.