Follow this link and input your car year make and model and select "heater core replacement". It will give you a average range of what the labor cost and parts costs are for your zip code. http://repairpal.com/estimator/results
The Alldata labor guide shows 5.2 hours to replace the heater core in your car. There are obviously many factors that can affect the actual time, including your knowledge, access to the proper tools, potential rust issues, etc. If you are planning to perform the repairs yourself, make sure that you thoroughly flush the coolant system before you install the new part, to ensure that any contaminants in the coolant system don't contaminate the new part.