Ahhhhhh then he received a WOK melding probably
Depends a little bit if there needs to be any welding and how much welding etc. But I would count on 200-250 euro or something on average to mount/weld something in between. I believe common prices for adding a valved muffler can be quite costly (so material + labor). For a cutout, prices are about 400-600 euro for a single cutout valve (material+labor).