Depends on the parts.
We just put a full fabspeed system onto a 996 Turbo, faster spool up, at last a soundtrack to rival a F360 with a tubi, no fault codes and much more power.. But about £4k
We have fitted a lot of stainless aftermarket systems we get from Germany, audio improvement, slight improved throttle response, possible power gain, more so if its a complete system with manifolds and cats, but not tested it and again no fault codes.
Have fitted some aftermarket systems customers have supplied from various UK suppliers... fault codes, poor soundtrack and who knows about the power.
With the 996 it is widely misunderstood as a engine management system, a place about 8 miles from my workshop in Poole has seriously buggered a few Porsches trying to map them, a Bosch service centre with some basic mapping equipment has destroyed a cayenne, made a mess of some 996's, and has now decided to advertise as a Porsche specialist!!!! WTF!?!?

The problem is, most mappers just alter the maps, often via the OBDII port... A mapped car has narrower margins of flexibility, which means that an exhaust change after a mapping, or a bad exhaust design, can really make the engine management system throw a fit... Unless you alter a few parts of the software, or understand the exhaust backflow issues and can alter the error margins, which sometimes also needs an alteration to the software.
There are also a lot of people who franchise map, which means they pay a license fee and squirt a pre designed off the peg map into your car... Problem is, you get poor increases and something to generic to get good gains... The other problem is, some of these off the peg maps are designed for other markets where emissions are stricter than here, so often you can get a better map by making it UK/Europe legal, rather than having it have to pass smog regulations in california when the car will never go there... We tend to prefer to custom map each car.
The same idea is behind why the better aftermarket exhausts produce gains, which is because they will pass a UK MOT without issue, but would not pass californian smog regulations which are performed on a rolling road, where as in the UK it only has to pass a standing test... Therefore a less restrictive cat and system is required, the lambda sensors can tolerate a wider acceptable range (see reprogramming) and you get more power without reduced reliability.
Price for an aftermarket exhaust, all depends how much of it is to go.. just the back boxes or the whole lot?