Well, the coil pack should have been very cheap to replace - under $100 for parts and no more than 30 minutes labor - probably no more than $125 total (plus diagnostic). The heater core would be much more… but if the mechanic knew the shortcuts for the car that don’t require taking out the dash, it could have been much less.
But my question is why is the heater core is being replaced? That would be unrelated to your other problem. These cars don’t have the greatest cores, but if the fluid is changed regularly, it shouldn’t be an issue.