Costs vary massively between vehicles, smaller vehicles with petrol engines are the cheapest to replace, the exhaust system, minus the CAT will typically cost less than £100. Larger vehicles with larger engines and more complicated exhaust systems can cost considerable more.
When it comes to CATs (Catalytic Converters) and DPFs (Diesel Particulate Filters) then these are typically more expensive. Again, they do vary for engine size (some cars have two of each) and some are combined units, and / or integrated into the exhaust manifold casting that is attached directly to the engine block. These can cost from a couple of hundred pounds to several hundred; depending on the size, make and design.