I'm sure you know this, but you will be poor for 4 more years if you get into medical school. And you will still need professional clothes, and will be judged on it. No need to overspend as a student, but you DO want to start cultivating a professional wardrobe.
This.
I get the budget thing. I took out so many loans it made my head spin. I never came from any money whatsoever in the first place. Living frugally has just been what I do for so long.
That said, dressing well absolutely doesn't have to cost much. Consider what I wore to residency interviews:
1. Navy Calvin Klein suit from Marshalls - $159 (minimal tailoring ~$20)
2. White Jos A Bank tailored fit "traveler" shirt with spread collar - ~$30 on sale (I don't like Jos A Bank, but the tailored fit traveler shirt had great reviews as a shirt that could stand up to the rigors of interview season and never really need to be ironed if I didn't have time)
3. Navy wool socks - $3 at Marshalls
4. Burgundy Johnston & Murphy Melton captoe balmoral/oxfords - $29 very gently used on ebay
5. Burgundy pindot tie I've had for years, can't remember where or when I got it
A grand total of just less than $250 was spent on that ensemble, and I still don't mind saying I was probably the best dressed applicant in most rooms. The suit looked great with just a little waist suppression, and the cap-toe Meltons are a great looking less expensive alternative to more costly shoes (even more so when you buy a pair used online, and they're all over ebay). On the interview trail for residencies, you could definitely pick out the people who at least had a decent idea of what they were doing among all the people who, although about to graduate from medical school, had still barely graduated from dad's striped sack suit with slip on Rockports.
A little attention to detail (well, and the big picture) can go a long way. I am not at all saying any of this helped me in a way that dressing slightly worse could not have; however, knowing I was well dressed at interviews certainly helped me project confidence. And I realize not everyone prioritizes fashion and such, but some of this is just very basic stuff that should be reflexive and intuitive for an adult male (get a suit that fits, laced up shoes [balmorals if possible], polish your shoes, avoid brightly colored shirts, etc). You'd be surprised at how little effort it actually takes (once you know what to do) to take your appearance from "I have no idea what I'm doing in these big boy clothes" to "I make Barney Stinson look like Barney the Dinosaur." The deeper nuances you can forget about if you so choose. But at least try to take some of the basics to heart, if you care at all about the way in which you present yourself to others professionally. Whether you like it or not, appearances count for a lot in any line of work.