I'll try to answer your question in two parts. First off, the reason salt was such a valued commodity was it's many practical and often strategic uses. The primary application of salt was for the preservation of foods before the invention of refrigeration. This may not seem like a big deal today because our food often comes preserved for us, but imagine having to constantly source fresh food or risk it spoiling quickly. This becomes especially important for stockpiling large amounts of food/planning. Meat can be cured using salt to cut back on the number of animals you need to bring with you, as well as making more efficient use of leftovers. Vegetables can be pickled in salt for use in the winter/outside of normal seasonal availability. Thus controlling the salt largely controls the food, without which you won't be fielding any armies or making many trips. This utility was so universal that we still use words based on salt like "salary" or "soldier". It is also why Jesus referred to his followers as the "salt" of the earth.
On to finding the salt, even right next to a sea full of it: While the Mediterranean is indeed full of saltwater, the process of gleaning salt from the ocean is extremely labor and time consuming. The Romans would boil their seawater in giant pots, but this yields relatively little usable salt. The Romans even had a special highway called the Via Salina connecting Rome to the opposite side of the boot. This was because the Adriatic has a larger amount of salt in the water compared to the shores near Rome. Brine pools and solid mineral deposits are much more lucrative sources, but are obviously more geographically limited.
I would highly recommend Mark Kurlansky's "Salt". It offers a surprisingly fascinating history of one of the driving forces of history, from ancient China until the industrial revolution. His book includes a whole section on Roman salt!
The Medieval version of this question was asked a few weeks ago: https://www.reddit.com/r/AskHistorians/comments/d1qgt6/i_know_that_in_the_middle_ages_salt_was_an/
The Roman answer (which is briefly mentioned in my reply to the Medieval question) is the same: salt wasn't super-valuable. Where salt was cheap (i.e., near salt producing areas, or where it could be transported to from such places cheaply, and where taxation didn't make it expensive), the price was about the same as the price of wheat.
Why not free, or at least much cheaper than wheat? This is answered in various past replies:
u/wcspaz in https://www.reddit.com/r/AskHistorians/comments/4kz3le/im_given_to_understand_that_in_the_premodern/
u/Lynx_Rufus in https://www.reddit.com/r/AskHistorians/comments/5opntw/why_was_salt_so_scarce_and_valuable_when_it_is_in/
Basically, while seawater is free when you live on the coast, it still takes time and effort (and therefore labour costs) to extract salt, so salt isn't free.