A couple of years ago I had a new stainless steel custom made centre silencer box made, oval shape to give better ground clearance than the usual round shaped one as the car is quite low. Cost me £680, so I can see a complete system custom made would cost a lot. My car was there all day as there was nothing off the shelf that they could use. It is a skilled job to custom make an exhaust system and that will be reflected in the price.