Shipping is usually the reason for higher cost. If you can find some within driving distance it's usually worthwhile to get those. I bought some last year from eBay for $100 shipped. You can also use a Mach 1 catback, which show up from time to time on Mach 1 sites.
Or you can check the classifieds here on stangnet. I found one here off a 2004 GT with less than 200 miles on them and bought them for 60 bucks installed them and am very very happy with them