Suspect, what model Rocket are you working with?
If Standard or Classic, then D&Ds are your lowest cost option. Jardines, while more expensive, are VERY loud with the benefit of some increase in power. The Triumph cat-bypass and TORs (Triumph Off-Road exhaust) are also loud but not my favorite sound. Staintunes are not as loud but have a great sound to them. Unfortunately, they are somewhat expensive. Other custom options are very expensive.
I can't comment on the Roadster and Touring models.