Really depends on where you live.
Looking at your profile I see you live in NY...
Well I would think $300-400 tops.
If its more then that go to
Parts for your car
They hav an SLP kit (which is nice) for around $450 !
It uses the Flowmaster 80series with 2.5" in and dual 2.5" out.
Or borla but who has $700 for some shiny pipe you cant even see
When I did a custom exhaust on my SE, w/headers and high flow cat, I ran 2.5" through out. It sounds better every day