The reason this pipe is $900 is that it includes a catalytic converter. If
your vehicle has under 80,000 miles on it, it should be covered by the
federally mandated emissions warranty (presuming you're not in
Factory catalytic converters can rarely be had for $300. The primary
reason for aftermarket converters being less expensive is that they have
less of the precious metals that are necessary for the catalyst to be
effective. This results in either a less effective converter or a shorter
converter life, or both.
If you can find a used one in good condition that's not a bad option. But
with a 2002 vehicle, unless you have a tremendous amount of mileage, I'd
recommend against aftermarket. If you go with aftermarket, you're likely
to have more problems down the road.