You need to locate the source of the leak or its cause before you can know the price. It could be a simple fix such as tightening a hose clamp or worse case scenario, a bad cylinder head gasket or even a cracked cylinder head if the overheating had been bad.

When the coolant was replaced, was system bled of air? Top up the coolant and check for leaks by noting if there are any coolant stains, especially around hose joints. A pressure test might be necessary if the leak is small.