is it a gt or a v6??
either way the only thing you can really do is buy some aftermarket mufflers. I love the Hooker mufflers. They sound great and there very high quality. But you cant really go wrong with name brands like pypes, slp, flowmaster, etc...
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-2000 Mustang GT Ford Racing 4.10s // T45R 5speed// Hawk HPS Brake Pads // Cold Air // BBK 75mm TB // BBK X-pipe // Hooker Aero Chambers // Eibach Sportline's // Ford Racing Upper and Maximum Motorsports Lower Control Arms // '95 Cobra R's // Toyo Proxes T1-S // Steeda Tri- Ax Ram HDX Clutch // tuned w/ SCT X3
for that kind of money newtakeoff.com would be your best bet to fit close to your budget or look in your local craigs list ive seen several dual exhaust kits for like 130
I got a GT muffler takeoff from ninosports for $70 after shipping. I felt a suttle difference in power (people said it was about 8-10 rwhp) but it also sounded much better. Im pretty pleased with the route i took. Id def recomend getting a gt takeoff
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2005 Sonic Blue 4.0 Mods:
Shaker 500 radio
GT rear sway bar
GT Muffler
C&L CAI/Xcal3 combo(91 bamachips Tune)
Want to get: 4.10 gears(w/t-lok), brakes, possibly screamin' demon coil
first thing i did was buy a set of gt takeoffs, got them from ninosport too. ran one till i could come up with pipes to go dual, at which time i put the other one on. i like em, they are not obnoxious, and come alive when you nail it. you could find used ones pretty cheap, but the takeoffs are like brand new. 2 mufflers for less than the price of 2 muffler hangers is not bad, and they fit right on. the tips will stick out a couple inches cause they are straight instead of curved like the v6 muff, but not bad.
exhaust first upgrade 
