Obviously you have been so screwed by your ex. As you stated, it's the money and only the money. Any decent human being ( the ex ) would acknowledge their mistake and not make you pay for it...but her actions are apparent. You can hire a lawyer and fight it...maybe settle on something
However, you need to at least get the guy in jail to submit to a DNA test so you can demonstrate he is the father. Maybe he will get out soon then take over...tough one and really sorry