Why do designer clothes cost so much?

As to your question of why designer clothes cost so much, it is a combination of things. First of all, to some extent you are paying for the name. Designers sell their clothing at higher prices to make them exclusive so not everyone can afford to buy them. They are not designing for the masses - they are designing for people who can afford to pay more for their designer clothing, bags, etc. You will usually find, however, that designers use better materials and fabrics for their clothing. If they are going to bother designing clothes, then they don't want to make it out of some garbage fabric. Also, I find that designer clothing is very well cut - and nothing will make your body look better than clothes that are cut well and have beautiful buttons and embellishments.

As far as how to become a designer, you can go to fashion schools which teach clothing design. Then you get a job and work your way up to becoming, hopefully, a designer. This takes time and effort so work hard.