What kind of cat food should I use?

I have been feeding my cat 9 Lives since I got her a month ago, and she has had perpetually runny stool, smells bad (in spite of her diligent hygiene and bathing habits...) and eats about 3 times as much food as she should need. She is not overweight, she is very small. I think she's eating so much because her body is not getting the nutrients it needs. I went to the pet store today, and asked for a recommendation, and the kind that came recommended was $24 for a 6 pound bag. Honestly, I would be willing to put her on something that expensive to see it improve her health, but my roommate and I split her expenses and she doesn't think it's worth it. Does anyone know of a good compromise? Something that's not too expensive but is high quality? I think we could settle on about $10 for a 6 pound bag.