An O2 sensor from the dealer will probably be $150 + or - and I was quoted like $776 for a cat. for my 95, which is complete crap. You can get an aftermarket Magnaflow cat. for $72 from SummitRacing.com if you had to.
Very rare that cats. go bad, though it does happen. And to everyone else's point, you can get something like a Denso (good stuff) O2 sensor much cheaper and install it yourself to see if that fixes the problem before you get into a new cat. I put a new Denso O2 on my car not too long ago. It's easy, it just may need a little persuasion to break loose.
And be sure you don't run lots of fuel and carb. cleaners and all that crap on an ongoing basis through your car because those are bad for your O2 sensor and cat. in the long run.