A new catalytic converter from Toyota is around $400. I think letting my van misfire and a blown headgasket is to blame for my catalytic converter to clog up a bit. My engine's been rebuilt. So, I'm looking forward to seeing 400,000-500,000 miles on the odometer. Oh wait. I'm NOT! I want this thing to last forever! Which it will.
But if it ain't broke. No need to fix it. Fix any misfires immediately and your cat should last a long time without replacing. I'm running Bosch Platinums now. I want to go with NGKs to be on the safe side.