Probably somewhere around $75 - $100 if you were to purchase one at an auto
parts store such as Advance Auto or Auto Zone. Some cores are easier to
replace than others. It may require a lot of work behind the dash. The core
in my wife's dodge went south and we were quoted $425 for parts and labor.
Since the car was on it's last leg, I decided to just bypass the heater core
by rerouting the hoses. Took all of 20 minutes to do that....the hoses that
come off the core are easily accessible under the hood. You're left with no
heat/defrost, but it got rid of the smell and steamy junk entering the
vehicle which would fog up the windshield and left a greasy film. BTW....the
*steam* is a little hard to see..at least in my case it was... you have to
look closely at the vents and watch for it. If money is tight and you want
to put the repair off for a while IF you find that it is indeed the heater
core, that's about all you can do for a temporary solution. I'd suggest a
haynes manual or whatever other repair manual to guide you if you want to
replace it with a new one yourself if that's the problem. Here is some basic
info about replacement

http://autozone.com/az/cds/en_us/0900823d/80/04/f4/ad/0900823d8004f4ad/repairInf
oPages.htm