go to an exhaust shop and ask for an estimate. heres mine; 2.25 inch exhaust from the cat back, USING my old resonator, and a 40 series flowmaster muffler. 150 for the piping, 62 for the flowmaster. grand total of 212 for a very nice sounding exhaust and no power loss. youre not going to gain very much useable power from a catback alone... have you looked into a header as well? hotshot makes a 4-2-1 header that will give you low to mid range power for about 350 bucks...
www.southwestautoworks.com is having a sale on them right now.