Are you specifically looking for used? Also what year and trim level challenger? (cat back exhaust diameters are different between 5.7 and 6.4) -- Nevermind I see you have a 2013 Challenger R/T. Is it an auto or manual -- again exhaust varies auto = 2.25 and manual is 2.5?
If you call vendors there is a good chance you can score a good deal.
I waited until last December when all the vendors offered systems at 20% off and free shipping.