Aftermarket exhausts can give your bike more kick - but not alone.
A stock exhaust is most restricted near the end, and the system overall is designed to restrict the flow of exhaust for two effects - lower horsepower, but more torque.
If you are trying to get more power out of your engine (and it's not as simple as that) you need to replace the full exhaust system, including the headers and the pipes, and reject/tune the carburetors correctly. Yes, all aftermarket exhaust systems are going to be louder. I'd recommend youtube - there are a few nice videos demonstrating exhaust sounds/comparing stock to aftermarket. Some of them I've even seen them use a DB meter to show you the difference in levels before/after.
The only way your going to get less than 300 bucks is to use slip-on systems (which can easily cost as much as full exhaust systems in some cases), but your looking at MAYBE a 2% increase in power, more than likely none. but it will change the sound if that's what your looking for.