The reason those exhaust systems are so expensive is that they are engineered through a good deal of testing to provide the right back pressure and vacuum at the exhaust outlets while reducing weight, fitting up perfectly and looking good. Then comes the brand name.
Feel free to skip the next few paragraphs if you don't care about the why and the how.
The waves of pressure in the exhaust gases and the sound waves influence the engine's ability to vacate exhaust gases from the cylinders. When engines develop their best horsepower and torque at a certain RPM range, it has to do with this phenomenon as well as a variety of other factors. At any RPM, there are negative pressure waves (vacuum) occurring at the exhaust outlets, as well as positive pressure wave, like the sucking and pushing of an ocean wave at the beach. Engine tuners will design exhaust systems so that at a certain RPM range (based on personal preference) those negative pressure waves are synced with the exhaust valves opening so that they suck exhaust out of the cylinder.
This allows for valve overlap. Normally the exhaust valve is closed during the intake stroke and vice versa. However, we now have a force sucking exhaust out of the cylinder. Combine that with the fact that the hot exhaust won't mix well with the cold intake and we can now keep the exhaust and intake valves open simultaneously without exhaust accidentally going out the intake valve into the intake manifold. Basically, the engine can "breathe" better, which is a term you'll hear people say a lot. Increasing the flow of gases per second allows greater power output.
You've probably seen "VTEC" written on the side of a car more than once. It stands for variable timing exhaust cams. The timing of the cams in this case is synced to the pressure waves in the exhaust system at a range of different RPMs, which essentially makes the power curve straighter.
----------------------------
To be clear, any exhaust that you buy from a major company is going to be better than something you make yourself unless you are an engineer. If you want to go cheap, you WILL get cheap results. That's not to say that a slip-on and a set of custom headers isn't going to work ok for you. The rejetting is best done by a professional; it's not easy to do right and if you want your bike to run optimally, expect to spend many hours of trial and error until you get it right (maybe) if you plan on doing it yourself.
My recommendation is going with a name-brand slip-on or full system and then taking it to a shop to get rejected professionally. The results are worth it.
