My take on all this, whether man or woman (and GP, feel free to jump in) is that it is not the label, per se, but the overall quality of the look. Most importantly, FIT. You could have the most expensive Saks suit on, but if it is tight, too long, etc., that will detract from your overall presentation, and get you remembered in a bad way. For instance, there is a guy at my old firm and whenever I refer to him (partially because I can't remember his name), I always say "the guy who worked for Joe whose pants were too tight").
When shopping, look for quality. Something that will last. You can find bargains, especially these days, with sales, etc., but make sure it is something that you can actually wear. The easiest would be a classic navy suit. Make sure it fits your CURRENT body (not your body after you start going to the gym every morning at 5am and finish boot camp). Spend money on tailoring - very important. I don't know why people skimp here. Get the pants and the sleeves properly tailored. Bring the shoes you normally would wear with the suit or outfit to the tailor.
I worked with a partner when I was a first year associate. He was reportedly independently wealthy and also brilliant. However, the main thing I noticed about him was that his suits were just old looking and too tight. I thought at the time "we all gain and lose weight, sometimes we just need to accept what doesn't fit and put it away...or give it away. Sometimes we just need to buy new suits." [or other business attire].
A related topic here is to make sure that the clothing sits right on the body. Now, this tends to be more of a woman issue, but the gap in the blouse in the middle of the chest, the shirt that won't stay tucked in, etc., also detract from your look. "Too sexy" clothing further cuts down on the professionalism. You don't want your nickname at the firm to be "the half naked one." Similarly, undergarments are just that...things to be worn UNDER clothes - you don't want thongs showing in the AMLAW -- yes, we have seen this.
Overall, it is not so much the price but the overall quality and fit of the clothing. You want to project a professional appearance. You want to be remembered for your stellar work, your work ethic, your collegiality. You don't want to be remembered for your tight suit pants.