[AWDFTW]

I think your mechanic is wrong and whaty he told you is exactly the opposite of reality with our cars (I'm assuming you have an Outback. Year? Miles?)

P0420 is VERY RARELY a failing cat.

It is most commonly a failing oxygen sensor (front Subaru, rear any brand), vacuum leak, exhaust leak or what not.

Search that code on the forums and you'll find plenty of info.

Cats rarely fail without serious mechanical problems, I think your mechanic wants to give you an expensive repair bill.