I paid 35k for mine - 2015 S4 with 24k miles. Still under factory warranty. The add ins here are great but, as Tim said, there is no reliable fix for the cats unless you go catless. I have not been in one without cats but have to believe it is going to drone as the stock exhaust can drone a bit at certain speeds / rpm.
That being said, 2 sets of wheels and runs low 11s all day? Not a bad deal. I have to admit this one is probably even cleaner than mine.