You don't have to pay for extremely expensive interview clothes, but you do want to choose high-quality clothes with little or no wear. Ideally, you'll have a couple of outfits set aside only for interviews.
This way, you become comfortable wearing the clothes, but they don't get worn out with normal use. You want everything to be as starched and clean-cut as possible.
Pick a high-quality dress shirt that comes with metal collar stays so that your collar will be sharp and crisp. Don't settle for cheaper plastic collar stays that don't work as effectively.
Be sure that the fit of your clothes is perfect. This is not the time to wear that pair of pants that are a little bit too loose for you or that dress shirt that has a tendency to come untucked.
Everything you wear to your interview should fit you perfectly when you're sitting and standing. It's best to choose pants that don't require ironing so they won't wrinkle while you're waiting for your interview.
A shirt-gripping waistband makes it much less likely that your shirt will become untucked at the least opportune moment.