The first thing you need to know is why it's bad. Catalytic converters are pretty tough. I've had the same one for almost 20 years now and have never had to replace it.
There are really only 2 reasons why it would need to be replaced. The first is that it hit something which damaged the honeycomb like interior, making it stop doing it's job.
The second is that raw gas is being dumped into the converter and burning there. When this happens the burning gas destroys the converter. If this is the case then you need to fix the gas problem before you replace the converter. Usually there is a faulty fuel injector which keeps dumping gas even though it's not being burned. This means that the "raw" or unburned gas is going out your exhaust. The Catalytic converter is so hot that it burns it there. Unfortunately this is not the job of the converter and it can be destroyed.
Get a second opinion and find out why they think it's bad. They are pretty hard to damage. Also know that most of the cost should be the part, not the labor. Hope this helps.
〉 Answered on Dec 5th, 2007 by Lori Johnson, Owner and Instructor at Ladies Start Your Engines