Separates are the best budget option and they don't have to look boring. If you've got an unusual shaped skirt, or an interesting shirt, you can mix and match easily. All you need then are the accessories. They're the easiest (and cheapest) way to brighten up an outfit. Think jewel colored patents and sleek black to make your outfit look more expensive than it really is. Structure looks smart, floaty can look unprofessional.
Of course, if you really need to impress, you could hire a dress. No longer is dress hire something to be sniffed at for country debutantes. Sites like Girl Meets Dress have made designer dressing easy. They'll deliver a stunning dress, you wear it to your interview, and then pop it back in the post. It can cost as little as £25 for two days so it's a good option if you're really in a pickle. Just don't spill anything on that Fendi.
Just remember, interview preparation goes a lot further than deciding what shoes to wear. A good night's sleep and a smile won't cost you a penny.