Last year my old E39 BMW failed its MOT on the same thing. I put a new cat on it, dropped some Forte in the fuel and took it for a good run then back in for the re-test and it was perfect.

A brand new, non-genuine cat was £90 with a 25% club discount I had.

BMW parts are dirt cheap though if you stay away from main dealers as there's just so much choice. I bought an air suspension unit for the same car for £110 and it was a fully stamped genuine BMW part, but the main dealer wanted over £600 for the exact same part!

Jap cars aren't as easy to find these bargains for in my experience, but do some research, there are cat specialists out there and some cars can use a ceramic cat rather than a precious metals type and are therefore MUCH cheaper. They have a lower life expectancy, but should still last a good few years and you should be able to but at least 5 for same price as a genuine one from a dealer!