Hiya,
To answer your question, go to longlife.co.uk, they quoted me £299 for a stainless system from the CAT back (not inc cat), but Fiat wanted more like £700. Longlife also said if I wanted to de-cat and change the whole system it would be just short of £400! Fiat want nearly £800 for the full system because the cat is part of the centre pipe!
Steve