Personally, I'm just throwing something together from my wardrobe, not a fitted suit or anything. I've got an old black blazer that I found on a clearance rack for $8, a knee-length black skirt from K-mart which was something like $5, a cheap pair of black pants... you get the point.
Does anyone think it's a poor decision to not look... I dunno, expensive? I mean, I'll look nice, but not Express or suit that was all bought from one store nice.
Of course, I'll probably get in to school on the "poor kid raised in a bad neighborhood, comes from a zero-income family, divorced parents, first in the family to go to college, has to put herself through on her own with lots of loans, supports herself working a $5.55/hr job because it's the only thing that'll work with her school schedule BUT has overcome everything while maintaining a high GPA and staying out of trouble" appeal... maybe looking like a poor kid trying to clean up will be to my benefit.
I would recommend looking in a store that gets its clothes from other stores (like Marshall's or Gabriel Brothers or Ross Dress for Less or anything along those lines, whatever you have in your area.) I've actually found that they tend to have a lot of clothes that are on the smaller end of things simply because there are way less size 4s than size 10s (at least that's how it is here in West Virginia, third fattest state in the nation.) You can find the stuff from places like Express if you're willing to make a couple trips over a few weeks, except it's tons cheaper.