You won't find a cheap, quiet 2.5" exhaust in the aftermarket. The extra resonance created by piping that big will create a much louder exhaust note. Also, a chambered muffler would quiet things down a bit, but tend to be much more restrictive than a straight through design.

If any of that doesn't bother you and you are willing to spend a little for quality, look into the T1R catback. It features 2.5" piping and a pretty sick muffler. It is a little pricey, though.