For a cat back under $300, you might get lucky with a used system, unless you know some body with a stick welder and some extra scrap odds and ends of pipe laying around to put a cherry bomb on

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New, you won't find anything for 300. A group buy deal on a Raptor Racing kit (about as low price of the best quality systems) was something like $600 last year as I remember, and from the Japanese tuner houses, they start at something like $700 for the old school HKS Turbo (and that is not a stainless kit...it will rot through eventually) through to the HKS Dragger (which is stainless) in the mid $700, and then it gets all crazy and prices top $1k from there depending on what you want...This is all catback pricing. Add $150-200 for a 3 inch cat, and a few hundred more for a downpipe!