I'm assuming you mean intake/headers/exhaust? I'm still a novice in some aspects but basic bolt ons wont yield big gains with the 350Z considering the engine is pretty much maxed out.

Intake could be installed yourself and expect between 3-8 wheel horsepower gain

Headers will be the most expensive to get installed (Around $300-$400) by far and yield 5-10whp depending on which brand.. some claim 20whp gains but uhm.. not many people can show a dyno to prove that. Nismo Headers cost around $1,000 and the cheapest I've seen are from Top Speed for around $300.

Exhaust .. I've seen 8 to 15hp gain. Nismo is really good and HKS is one of the best but one of the most expensive by far. Nismo exhaust is around $900 new and the HKS is $1400.

At one of our dyno days the guy that put down the best numbers naturally aspirated had the Injen CAI, Borla Exhaust, and Crawford Plenum, UR Pulleys and ECU Reflash.

Hope this helps