Heater Core?

Thanks for the other answer.  Now I think my 2009 Liberty 4x4 has a heater core gone?  Not only does it smell inside the car, but has like an oily stain on the side of the passenger side under the dash on the carpet, plus a wet spot under the foot carpet.  And, on my way home this afternoon, car is starting to over heat?  Have nothing leaking onto the ground though?  What do you think, and what does this cost to repair????? Thanks