2000 Ford Escort Antifreeze pouring out of muffler

I drover to work, parked my car, went inside for about 20 minutes, went back outside, tried to start the car, it wouldnt start. It would turn over and try to start and if I held the gas all the way to the floor, it started but sounded like metal clanking. I could smell antifreeze. I checked the reservoir, oil and transmission, all seemed ok. Then I looked at the ground and streaming from the hole in the muffler was greenish yellow liquid which I'm assuming is antifreeze. What is wrong and how much should it cost?