Close to $1400 from the dealership when mine went out, so I went looking for another option. I went looking for aftermarket cats that would work on my car and someone in a forum mentioned a certain model magnaflow cat that would work in these cars. I tried it in mine and the cat code came back two days later. Junkyards are not allowed to sell used cats to people around here so that option was out. I just ended up driving the car with the light on sinse then. I don't have a problem with inspections where I am and I'm still on the lookout for parts cars that may be able to help my situation.
Some info that may be useful, these german cats have alot of prescious metals inside them and if you happen to find a recycling center that buys used cats, they should give you about $80 per cat, so if you do change both with aftermarket ones and you trade in both of yours, it should even out the costs.
You should just change the one that is busted with an aftermarket one, and if the light doesn't cut on for at least one week, then you can be sure that the aftermarket one will work with this car, and then change the other one if you wish. Or pay $1400 from the dealer, either way, I would ask the shop to give me the old ones back, either to recoop the costs, or to hold onto just in case the aftermarket ones throw a code later on.