ouch! I just had that happen to my car this past week, and that will be only one thing: Catalytic converter needs to be replaced, as well as you better get a tune-up on the car, since one of the spark plugs leaked. That is not good for your car and it will cause your car to sputter and putter down the road until you get it to the dealership or local mechanic right away.
It cost me about $400-600 for the tune-up, and the catalytic converter will cost you around $800 or so depending on where you go to get it done at. Don't get these 2 things done at a gas station...they will rob you blind with a lot of money to pay for something that could be drastic and major later down the road.