If you took a cashmere sweater from J. Crew and a cashmere sweater from Loro Piana, the everyday shopper probably wouldn't notice much difference, except in the price of course. But to those who know, to those who appreciate the art of fine knitwear and fantastic design, one would instantly recognize the incredibly fine gauge. The higher the gauge, the finer the knit.
Same with a trench coat from Burberry and a trench coat from Banana Republic. Burberry, the creator of the most iconic outwear, includes phenomenal fabrics, visible darting and pin tucks for the perfect, tailored fit, and backer buttons (those little buttons on the reverse side of outer buttons that provide addition strength and durability) for additional quality. You'd be lucky to find any of these details in the cheaper, more mainstream version.
Price isn't everything. You can find great style, quality, and design at a bargain as well. But the cliche is true - you get what you pay for. Design and details aren't cheap. And the most important part of each garment is always in the details.