I've been looking around for the same kind of thing! The thing is that most noise (from what I gathered) doesn't necessarily come from CASE fans.. but GPU and/or CPU fans. A stock CPU fan doesn't give a lot of noise (I think), but from your budget, I'm assuming you're overclocking and have a non-stock fan. Silent case fans are generally helpful, but not if the case has a bunch of holes all over the place that allow the GPU fans to emit a lot of noise that gets through.
To solve that problem, I've seen quite a few cases with foam padding on the inside. While that may be excessive, (Fractal R3) a good idea would be to get a case that doesn't have many holes (except for the ones necessary for airflow). Commonly, a lot of noise comes out of unused fan mounts. The R3 solves this problem by covering unused fan mounts. It apparently has fairly good airflow, as well.