In simple terms:
The main difference in cost is the material they are made from. Aluminized is cheaper than stainless. A good example is Gibson. The aluminized is $300s, the stainless $500s. They would perform equally, but the stainless will look nicer.
Most of the brands out there make Horsepower, but at the cost of low end torque. This means you will have better acceleration in mid to high RPMs, but less at low RPMs. Overall, slightly better than stock, which is typical for a V8.
In contrast, exhaust systems on some vehicles, like basically any 4 cylinder import, actually take power away. They loose torque over more of the torqueband than they gain power, so even though the HP numbers may read higher, they are slower accelerating. (And to me these "fart pipes" sound like chainsaws or RC planes, not sports cars)
The main differences between brands is the sound and looks. People prefer one sound or look over another. Power changes are so close that you will not notice any difference between one or another at the track or with the butt dyno.
Most of the exhausts come with similar price tags if they are made from the same material. Flowmaster is a little cheaper, but has a little cheaper manufacture look with seams on the mufflers, etc.
Other ones to look at:
Gibson
Borla
Corsa
Banks
Stillen
Magnaflow
B&B
All will give you similar power increases(within a couple HP) but all sound and look different, and have different prices. If possible, locate an armada or QX with a system you are interested in and listen to the system inside and out. This is the best way to see if its for you as the sound is the largest difference between them and the thing you will love or hate about the exhaust.
