I think there is a pretty substantial difference in quality and quality control, but the price does seem to increase exponentially. Occasionally, I hear about premium lines having a quality issue, whereas if contemporary lines have more issues. As well, premium lines use better materials and are more detailed oriented.
I own items from
Gucci, LV, Dolce & Gabbana, and Christian Dior and Burberry and the stitching is supreme (straight, no loose strings, consistent sizing, and the same number of stitches on each handle), patterns are placed symmetrically on the items, and the materials are resilient. For items I own from contemporary lines, I notice more threads out of place, there's no concern for pattern placement, hardware isn't aligned properly, fabrics and leathers aren't as nice to the touch, etc.
There are exceptions to this rule. My ten year old
Coach bag is top notch. On the other hand, there are some premium lines I just won't buy based upon quality issues. I can justify overlooking stitching issues on a $200 bag; however, I certainly won't do that for a $2000 bag.