Here is to give you an idea of cost from the Nengun Website. - http://www.nengun.com/
Different brands vary in different prices. Here are some examples.
BLITZ Front Pipe Nissan - Skyline ER34 AU $276.55
- http://www.nengun.com/blitz/front-pipe
BLITZ Nur Spec R Cat back will go for about AU $ 951.
Nissan Skyline ER34 RB25DET 98/06-01/06 2 door NUR-R, Pipe Size 80mm, Tip 114.3mm, 2 peices
- http://www.nengun.com/blitz/nur-spec-r-exhaust-system
Any High Flow Cat will do for afew hundred dollars.
HKS Front Mount Intercooler
Nissan - Skyline ER34 - AL Type - 600X301X65mm AU $1,673.53
- http://www.nengun.com/hks/intercooler-kit-type-s
APexi, ARC, BLITZ, HKS, Nismo and K&N air filters can range from $50 to $250, all do a similar job, just personal preferance...
- http://www.nengun.com/intake-panel-filters/