Suits are expensive, and there really isn't a way around them. It might be a tough reality to swallow, but expect to spend at least $100 on a suit set that you'll be able to wear again long after you've scored the job. When looking for a suit, be sure to make sure that you're buying the full set; even expensive suits can look cheap if the colors don't exactly match.
As for style, don't veer too trendy: Stick to a two-button blazer, and a high-waisted skirt or a slim-leg, cropped trouser (save those short suits and wide-leg styles for when you get your first bonus!). As long as the shoulders fit, the silhouette just grazes your body (not too tight or too loose!), you can have the lengths tailored pretty cheaply. Afterward, the blazer and bottom can be worn separately in a zillion different, office-appropriate ways. As for retailers, we're big fans of Express' suiting section for affordable pieces that won't make you look like a '90s telemarketer.
Express One Button Jacket, $98, available at Express.