The cost of fixing a coolant leak will vary significantly depending on the component that failed, in the first place. And, the severity of that leak. If it's a small leak or you happen to catch it and patch it up early on, it may cost you as little as $100. This would often involve replacing the many seals and gaskets where coolant might've leaked from. Or, re-sealing a small hole in the radiator. Even entire hoses won't cost you too much. However, in the case of more serious faults, such as needing to replace the whole radiator or replacing a head gasket, can easily cost you close to or upward of $1,000 or more. It's crucial that you find those leaks and fix them ASAP.