You'll be looking in the region of £300-£350 for a catback system. The 200cpi sport cat can be between £100-£300 on its own. A complete 2.5" S/S downpipe inc. 200cpi cat will cost in the regions of £550-£650. A full system from front to back can work out quite costly but it can make a fair difference.
I've had a few different combinations over the last 8 years.
2" Longlife catback system £374.
(As above) with 200cpi HJS sport cat and precat removed £300
2" S/S Hoffmann Downpipe inc. 200cpi sport cat £379
2.5" Milltek S/S catback system, group buy discount price £370
2.5" Custom S/S dowpipe inc. 200cpi HJS sport cat £575
4-1 dBilas manifold imported from Germany £260
Longlife replacement centrebox £90
Total: £2348
As you can see, it can certainly add up!