I had a custom S/S cat back dual exit system fitted to a 2.0ltr Cougar couple of years ago and it cost £300 but I supplied the two cans.
I belive the place I used quotes like this:-
Back boxes - from £90
Single exit cat back - £200
Dual exit cat back - £300
Manifolds fitted from - £100
Custom Induction kits including Filter - £80
Bit on the pricey side but he is a top bloke and does a really good job.
Most people will agree, if you can afford it change the mainfold for a 4-2-1 or 4-1 while the exhaust is off this will not only save you havin it all taken apart again in the future plus your see the most gains in performance from changin the manifold.