If your exhaust goes don't use these companies got quoted £400 from cb
; for new cat and £240+ from qf found complete exhaust on net for £ 230 including vat 
Mine field this!
Pricing depends where they source their components from. With exhausts, it difficult to know if you are getting good value for money or not. Some exhausts are very poorly made and the material very thin. Their service life will be shorter than a manufacturer that uses thicker, corrossion treated material.
Those exhaust manufacturers will charge a premium for their exhausts, but you can't be sure whether Charlie Brown for example is charging top money for cheap components.
One thing for sure is that they have to guarantee the product they sell you for a length of time. If that product fails, they will have to replace it and negotiate with the supplier. Their reputation is also at stake, so they will want to sell only the best made products to ensure minimum comebacks.
The exhaust suppliers usually charge your exhaust at retail and include fitting in the price, taken from the discount they get from their suppliers. But that may not always be the case as some may add a margin on top.
The prices for cat convertors especially can fluctuate greatly for the same vehicle from different suppliers. It could be that the components you bought on the internet are cheaply made, but they may not be.
If you bought the part from the manufacturers spare parts store as OE, then you would probably pay twice the price, but you would know that the part was good quality.