In our market we don’t have CARB but an OEM cat for a Mazda Rx8 retails at £1300. An aftermarket like for like is £800 (there are cheaper “race” cats but its a different product with less of the expensive precious metals inside).

Edit: a fairer example would be a ford focus - one of the most popular cars in the uk. £200 aftermarket main cat, £70 aftermarket pre-cat.