Heater core leak in my car. Cost to replace?

I understand that, but you probably just did a good $500* worth of damage by blinding cutting into the wrong area.

If I were you I’d at least get some quotes before you do more damage. Heater cores are notoriously hard to replace.

*Random guess, I don’t know exactly what you did, maybe a little less if they can patch it, maybe a lot more if you have to replace the coil, but your gonna cough up some money to recharge the system.