We just got in a used '11 G5(?) in at our shop. The stock muffler was looking like crap to me, so I decided to try a Emgo Dunstall style replica muffler. This is the 26" version. Muffler is right around 80 bucks.
The main thing that needs done is the muffler needs to be opened up slightly to fit over the head pipe. If you have a local muffler shop, take both the header and muffler and have them open it up so it just fits over.
I ended up wrapping the header with some pipe wrap, mainly because the stock heat shield isn't being used anymore. Also, if your like me, the rear foot peg mounting studs suck big time. Replace it with a 3/8" by 2" long carriage bolt and a lock nut.
The end result is a very nice mellow sound. Won't wake your neighbors up. All for under 100 bucks.