Does it smell like sulfur or rotten eggs? If so the catalytic converter is toast. Catalytic converters don't normally quit on their own. It's likely the engine is running rich (too much fuel) and even more likely the engine has an oil consumption problem from worn valve stem seals and/or valve guides. It makes no sense to replace the catalytic converter before you address the root cause.

How much? A catalytic converter is probably (wild guess) $800 installed at the dealer and half that at a muffler shop. Another $200 for an oxygen sensor that can't be too happy at this point. Valve seals can be replaced by an independent shop for around $400. Assuming your car has a 12 valve engine. Replacing the valve seals in a 24 valve engine will cost more.

Sixto
95 S420
87 300SDL