How do you know if the heater core is or has went out? any idea on how much to fix it?

If you have coolant pouring into the foot well, your heater core needs replacement.

http://www.youtube.com/watch?v=z8u3OpayY5s

make sure the "liquid" is actually antifreeze, because if its water, thats a completely different issue. another way to tell is to turn on the heater and if it smells sweet like antifreeze, the heater core is bad