I know it's an apocryphal story, but it's an interesting thought experiment about how much resources it would cost for Rome to do this.
You may get an interesting (if not historically accurate) answer on /r/theydidthemath
hi! you may be interested in the FAQ
if you're looking for hypotheticals, this is not the sub for that: speculation/conjecture are not permitted here. You can take that sort of question to /r/HistoryWhatIf
It's rather meaningless since salt is trivially cheap now whereas it used to be considerably more expensive.
If you're interested in Salt, Mark Kurlansky has an entertaining book on the subject.