I am in the same position as you LE. I have bought a few things, but they were inexpensive (a jacket from H&M for $15 and the Club Monaco pants for $9). Lots of sales are going on now in more "expensive" stores, and don't discount thrifting. If you live near a swankier town, you might surprise yourself with what you find. I will be going thrifting soon to see if I find anything.
I do not know what field you are in or how old you are, but I think it is highly likely that your wardrobe is more constricting than mine is (I can get away with a lot in my field), so I guess it's fairly easy for me. I agree with Angie about hair, shoes, and bags. I reserve certain shoes JUST for interviews because I want them to remain clean and polished (and of course I can't really walk in them in any other situation).
I guess the most important thing to keep in mind is that you don't have to get everything all at once. Step back and look in your closet. Imagine what you would wear if you got a call tomorrow and you'll probably find be able to put something, or even a few things together, but maybe something is missing. What is it that is missing?
What I really felt I was missing was a great jacket, so I got one. It would have made so many of the outfits I had in mind more crisp and polished. it is a white short sleeved jacket. I also need one with long sleeves for fall and winter, but since that time hasn't come yet, I'm not rushing it. I'm going to wait and see if NAS brings anything in my price range. If not, no big deal. Fall is far away. Angie mentioned dominant seasons. Where I live, it is cold for just as long as it is hot, so I really need a balance.
Next on my to-do list is a summer skirt. The only reason this didn't get higher priority is because I could wear my black lightweight wool skirt, even though it isn't exactly comfortable in the summer. Again, I'm trying to be patient, shop around, and wait to find exactly what I want. It's not an emergency. When you're on a budget, you shouldn't compromise and make regrettable decisions.