Because they've convinced women to pay that amount.
Studies have been done men vs women, and men pay less, while getting more product (say on something like deodorant) because it's easier to for marketing to influence women to be willing to pay more, while men will reject the higher price and go to a cheaper option.
So the better deals are with guy products which has had some women going to the guys section to get better deals, of they can find a product they like in that section.
But I can get a shirt and pants cheaper than you get a bra, and I assume it's a lot more material.
That being said, I don't know what material goes into a good bra and if that might be more expensive or not.