+1 HKS Silent Hi Power.
Got one on the R34, it is quiet but the note is super sexy and as stated before when you open it up, it growls like a mother f**ker. Loud under load, quiet at idle... perfect imo. I am running HKS front/dump as well for the record.
There was obviously a noticeable restriction reduction from my old Nismo system. It also looks great and is HKS so chicks clothes just fall off when they approach your car. As far as the cost, I can't remember exactly, it wasn't cheap but was reasonable.