Like wow and others have recommended, more people are going the inexpensive muffler shop route. HP gain is the same or more than any catback and sound is as good (but sound is subjective).
First replace the muffler with straight pipes. If that's not loud enough go catless or hiflow cats. Remove the resonators if you want every last bit of sound. Total cost less than $500.
I admit most of the catbacks sound very good but so does just removing stock muffler, cats, resonators. Borla, Corsa, etc., the muffler they use is what's mostly responsible for their sound.