I used to own a clothing boutique. I will tell you this:

1. I mark up 100%. I usually buy from wholesaler by the dozen.
2. Wholesaler mark up 50% from cos,t but they buy in the hundreds.
3. Rent and salary is the biggest expenses.
4. Trends change, this year you are the most popular, tomorrow your new fancy neighbor is the most popular. Well that is business. :D