catalytic converter repair/price/frequency on 2000 Mercedes-Benz ML55 AMG

My mechanic thinks one of my catalytic converters needs replacing. The car has 155K miles on it. How much should this cost and what is the likely hood the other 3 will break soon?

Asked by for the 2000 Mercedes-Benz ML55 AMG
Check out this advice concerning cats

Be sure that the diagnosis is very confident and that there is not another problem that will potentially result in more catalytic converter failures. You should be able to get an estimate from RepairPal: