How much to replace an entire foundation of existing duplex?

I own a 42-year old duplex. The foundation is in very bad condition right now from years of neglect and improper maintenance and drainage, etc. It may be so damaged now that it needs to be completely replaced. How much more money would it cost to simply replace the entire foundation instead of spending thousands of dollars to patch a foundation that may still fail at a later time? Obviously, it will cost more to replace the entire foundation than just repairing it but I would rather pay double for something I know will last rather than pay half for something that might last.