So, this is really a two part question. First, what would 30 tons of salt get me? And second, what would this sum be the equivalent of, roughly, in U.S. dollars. Like, would this be enough for me to live in opulence for the rest of my life? Or would it be more like I could live without working for about a year or two?
I'm a bit confused? Are you under the impression that at any point in the historical period salt was legal tender in the city? Because that's simply untrue, it's a misunderstanding of what salarium means
You may be interested in this similar post, where /u/bitparity comments
... Btw, a casual glance at the prices, shows salt as worth the same amount as a volume of wheat. So 100 lbs of salt would be worth its equivalent volume (or maybe weight?) of wheat. Hardly enough to retire on.
From this source:
http://www.constantinethegreatcoins.com/edict/
we get a price of salt as 100 denari per modius of salt. A modius is about 8 litres dry measure. 8 litres of salt weighs about 9.2 kilograms. 30 tons of wheat is 2,958 modi.
At 100 denarii per modius, your 30 tons of salt would be worth 295,820 denari.
An average general laborer would make an annual wage of 7,625 denari. An average Roman soldier would make 15,400 denari a year.
So, your 30 tons of salt would be worth about 39 years wages for a general laborer. It would be worth about 19 years wages for an average Roman soldier.
The average annual salary for a general laborer in the US is $29,648.
http://www1.salary.com/General-Laborer-salary.html
So the money you got for your 30 tons of salt then might be worth about the same as $1,156,272 today.
You would be quite well off, but not rich beyond the dreams of avarice.
By the way, 30 tons of salt today would be worth about $2,000 (if sea salt for eating – less than that if road salt for de-icing). The value of salt has fallen massively since Roman times.