How much salt was an average Roman soldier paid? Was it daily, just enough to season the evening meal?Did it vary by location depending on the scarcity of salt? Was salt a common means of exchange with merchants of the time?
The notion of Roman soldiers being paid in salt, or even receiving a salt allowance, is a modern fabrication. Salt was common, but not a common means of exchange. I wrote an answer a few months ago that explained how the myth arose: 'salt as salary' was invented for an 18th century Latin dictionary, and metamorphosed into a 'salt allowance' in 19th century Latin dictionaries. More recent dictionaries have erased both inventions.
The phrase 'worth one's salt' is a 19th century English expression, according to the OED. The earliest appearance of the phrase they know is an 1830 British naval adventure novel, here, at chapter 53.