Price has less to do with looking professional than fit does. Something that fits you well will always look sharp, and ill fitting items will look sloppy. Plain black or navy pants and a neat top, or dress (no overly loud prints), and a blazer will be good for nearly any interview. Go to the shops so you can try on different styles to find what works for you. Make sure it’s clean and unwrinkled. Lots to be found in op shops or target.