One question I get asked quite frequently is "What's the difference?" For instance, why one pair of jeans may cost $25 at one store and $200 at another. There actually are some differences but how people value those differences vary.
For example, Jantzen uses high quality fabrics and finishes on their swimwear. They also use good construction techniques. Their pattern-makers are highly skilled, so their swimsuits fit very well. So if you want a stylish swimsuit that will last for several years this is a good brand to buy. However, if you just want a trendy swimsuit that will last for the summer season, and you plan to spend more time around the pool rather than in it, than you may want to buy a low price tag swimsuit available at many discount retailers.
I remember when I worked at Gap, Inc. you could see costs differences between Banana Republic, Gap and Old Navy for similar designs. For instance, children's tee shirts in Gap would use embroidery or appliqués where as the same detail would just be a screen print for Old Navy. Another difference was types of fabrics. Such as Banana Republic would carry cashmere sweaters wwhereas Gap & Old Navy would have just cotton or wool.
Many of your boutique shops and designer labels use the best materials available for their clothes. In addition, there may be handiwork involved in creating their designs, such as hand sewing and hand dyeing of fabrics. Clothing may be constructed individually rather than factory mass made. Which means there is more attention to detail and less likely that an error in construction will occur. Of course, haute couture designers use the finest materials and custom-make garments for their clientele, with all sewing being done by hand. But just be aware, the majority of what you pay for designer brands with high prices is the brand name. In fact, a huge part of the mark-up on designer items is based on the fact that these companies know that customers will pay more for their fashion goods because of their brand recognition.
So the quality of fabric, notions (zippers, buttons, snaps), construction, fit, embellishments (appliqués, embroidery, screen prints, etc.), are all little differences between price points on similar garments. I should mention, the level of service you receive in the store is also a factor in the price. Self-service stores will by nature have lower prices than personal selling shops. Lastly, tor designer goods, a large proportion of the price you pay is so you can wear the brand name.
So what you value and are willing to pay for your clothing is up to you. If you are not going to wear the garment long, like seasonal fashion trends, than most likely a low-ticket item is what you need. Just remember the saying, "You get what you pay for".