There is a relationship between garment price and garment length. In general, higher-priced brands have longer inseam and skirt lengths than more affordable brands.
Is it because women in a more affluent demographic are taller than average? There is some research to suggest that taller people do in fact earn slightly more than shorter peers, but this applies more heavily to men and doesn’t work in reverse – meaning being tall might make you earn more, but being wealthy doesn’t make you tall. (If anyone knows of more research on this, I’d be curious to read it.) In any case, I don’t think high-end brands are spec’ing their inseam lengths based on this data. Height isn’t enough of a factor in wealth to justify this.
Is it because higher-end brands can afford to use a few more inches of fabric for each garment? This might be part of the reason.
The biggest reason I think higher price points tend to be longer lengths is because there is an assumption that the customer will get her clothes tailored. If your customer can afford a $300 pant, she can afford to take it to her tailor for a hem if she isn’t tall. The garment is probably well-made and worth investing a bit more into to get that perfect fit. For a $30 pant, there isn’t this same assumption, so the lengths need to work as-is for an average height woman.