It does not necessarily have to do with confidence. Not everyone who buys more fashion forward items lacks confidence. Many many things influence what we wear. Also, women’s bodies tend to be harder to fit, so a woman might spend more on a pair of jeans just because they fit well. She might try on 15 pairs and buy the one that fits almost regardless of the price. Jeans usually last a long time. Small waist, boy waist, big hips, no hips, big ass, flat ass, short rise, long rise (not even talking about low rise and high rise, which are more of a preference and not an actually body type problem) long legnth for higher heals, short for flats, etc. etc.
My Silver jeans (about $70) definitely make me look thinner than my Levi’s, and the denim, stitching, and dye is different between the two. I look neater and more put together. Women are judged more than men on how we look. Plus, some people just love fashion, just like some people appreciate art. Watch Valentinos The Last Emporer. It drives home his love of the female form and his garments are works of art. His focus on fit and how it accentuates the figure, he was truly incredible. A lot of cheap jeans have small back pockets, or misplaced back pockets that look terrible. Not Levi’s, but some others.