you missed one important thing, what size is the actual exhaust? the size of the tip means nothing.
$600 for an exhaust from the extraxtors back in stainless steel, which would include flex pipe, cat converter and cat back piping is a pretty good price.
look at it this way.. i have paid the following:
2.5" piping - $130
2.5" cannon with 4" tip - $220
2.5" cat converter - $200 supply & install
2 x 2.5" resonators (1x12" 1 x15") - $80 supply, $50 install
2.5" flex pipe - quoted $300 so its still on my to do list.
so far the exhaust has set me back $680, not including extractors which were $540 installed (rip off i know) and i'm looking at another $200-300 to finish it off.