Currently, I think $400 is about the going price. As Dave and Julie mention, the sound will mature if you're willing to wait and see if it grows to your liking. I wanted my exhaust note a little louder too. But I'm older and maybe my hearings just bad. So after 11 months I changed out the Bullitt axle backs for FRPP "stingers." As a familiar commercial says, "Can you hear me now?":lol:
Search for the best price you can find and then see if a vendor near you will do a price match. Some are willing to cut a little slack on price as long as they still make a decent profit. Pay attention to shipping charges. The stingers aren't real heavy, but will add to the cost if you ship a great distance.
The Bullitt sound is unique and I was very reluctant to make this change. But afterwards I'm very satisfied with the results. IMO they're not obnoxious. I also figured that typically, the cost of car parts doesn't get cheaper with time. Good luck.