To remove a centre silencer box would cost approximatly £40 - £50 but as tony correctly pointed out every car make and model exhaust is different so each car is assesed on its own merit and Autosafe Garage will provide you with a free estimate with no obligation, now you carnt get any fairer than that......
Welding the exhaust is sometimes not possible due to corrosion, you cannot weld corroded metal so it does depend on the condition of the exhaust that needs welding.