When the vehicle starts to get hesitant and (often) the engine light comes on, there is a good chance you have a plugged up catalytic converter. A mechanic or auto parts store can run the engine light and/or test the tailpipe pressure and tell you for sure, but if the converter needs replacing, it will cost you.
The price tag depends on whether you do it yourself or hire a mechanic and also depends on the type of catalytic converter you are replacing. Many makes and models have proprietary fittings. If yours uses universal fittings, however, you can save a lot of money and labor.
The average cost to purchase a new catalytic converter (you cannot repair a broken one or buy a used one legally) is in the $400 range. Some are much cheaper, especially on older vehicles with lower emissions requirements, and some are more expensive.
If your vehicle has universal fittings, then you can purchase a converter with universals installed and save a lot of labor and time. If not, you'll need to add universal couplings to a converter with no attachments and possibly to your exhaust pipes. Another option is to use joiner or flex couplings, if allowed in your state. You can expect to pay about $20, whatever your choice.
If you have a mechanic do the removal and installation of the old catalytic converter for you, you will pay half an hour to an hour's worth of labor, depending on the fittings (of course). They will have the tools to weld the pipes together properly, however, if that's needed.
In total, at the shop, with parts, you can expect this repair to run in the $600 to $800 range for most vehicles. Some will be much more expensive and others may be a little cheaper.