Like SuperUno, I got a complete system for my '86 FIRE (non CAT, front manifold back) for less than GBP 50. Check through the Yellow Pages and try and find a trade exhaust retailer. Don't use a high street Quick Fit type as they charge over the odds for fitting.
Most trade retailers will sell to the public, especially if you offer to pay cash.
Only downside is that you have to fit it yourself. Not a difficult job really, only problem is normally getting the rear box section flange to disconnect from the centre section. This has to be separated else you can't get the rear box over the rear axle.
However, if you're replacing the whole system you can cut the pipe with a hacksaw
Use proper exhaust gasket sealant, new manifold to downpipe gasket, rubber hangers and 'U' clamps. WD40 helps with rusted bolts. The Trade exhaust places will sell all these as well and they're not expensive.
If all goes well the job can be done in about a couple of hours or so.
Total cost for my Uno exhaust + parts was about GBP 50. The Kwick Fit places wanted well over GBP 100 to fit one.
Money saved!
Chas