Not Worth a Grain of Salt
This misstatement is likely caused by a mix-up between two common phrases: "worth his salt" and "with a grain of salt." The former refers to a person who is deemed of good stock and fine, upstanding character. Early Roman soldiers earned an allowance of salt for their efforts, and a soldier who was "worth his salt" was one whose performance was up to snuff. (The Latin word for this allowance, salarium, is where we derive the word salary) [source: Mussulman].
The latter phrase means to accept a statement while remaining skeptical about whether it's actually true. It refers to the idea that food is more easily swallowed when taken with a tiny bit of salt [source: The Phrase Finder].