I think your attire should:
1. Be professional - definitely a button up shirt, dress pants, dress socks, dress shoes (with laces). Lean towards conservative, white shirt, black pants if possible.
2. Fit well - not be 2 sizes too big or too small
If it's possible, I would highly recommend purchasing a suit second hand (~30 dollars) and getting it tailored (~45 dollars). Pick up a plain white dress shirt from an outlet store, or macys on sale (~20), and a conservative tie and DRESS socks(~20), and some laced up DRESS shoes (~60).
That is roughly 200 dollars, less if you already have some of the clothes. You can always borrow from friends/family as well, just make sure it doesnt "look" like it was borrowed (fit wise). This is an investment because how you dress subconsciously will affect how your interviewers view your professionalism.
When day is your interview? Feel free to PM me w/ a picture of your current attire and I will give you my honest (uneducated) opinion on how I think admissions will reflect upon it.