If you declare it as merchandise it actually pays a 35% tariff. After the tariff reform now being discussed in Congress is approved it will only pay 20%.
If you declare it as your personal luggage it would not pay duty at all, but the customs officers have the authority to ask you to open your luggage and to evaluate if the clothing is for commercial purposes or not, and if they consider that all or part of your luggage is commercial merchandise, they can charge you the corresponding tariff.
The value used for applying the tariff is based on their estimate, but there is a GATT valuation reform in process that will oblige them to charge you based on the commercial invoice. So travel with your commercial invoice if you don't want them to arbitrarily estimate the value of your merchandise.
It also depends on the quantity of clothes your are carrying and the time you will spend. For example, nobody will probably ask you questions if you come for say 30 days and bring 12-15 suits or dresses; you might be a socialite, a model or just dressy. But if you are coming for a week or are bringing in other gender than your's clothes in large quantities, you are tempting the devil.