Or just pick up up a used, stock take-off exhaust that someone is getting rid of. At $40 or so, they're the best bet if you're just concerned with cost.

If you go aftermarket then Ron Woods with Supertrapp muffler is the best for a 660 IMHO.

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'07 Rhino 660 * Mason LT * Elka Elites * Webcam, springs * Ron Woods * Pro Shop dyno tune * 38.2 hp at the wheels