I have often heard that gold and salt were sometimes considered of equal value. When and where did that occur?
During the era when the Phoenicians ruled the Mediterranean sea and surrounding territories (cerca 1550 - 300 B.C.), salt was indeed a highly precious commodity. After this, the Romans became the dominant force in the Mediterranean, though the value of salt did not immediately decline by any means. Whether it was pound-for-pound as valuable as gold, I think it is hard to say, but probably not out of the question at some point.
Until relatively modern times, salt was prized mainly for its ability to preserve foodstuffs as well as season food. In the hot Mediterranean climate where fish and meat would decay rather quickly, this was particularly useful. The period of Phonetician domination during the early to mid first millennium B.C. (including its successor state Carthage) represented one of the world's first monopolies. Essentially, other nations (including Ancient Greece and early Rome) did not have great access to salt, and hence the Phoenecian monopoly could dictate its own outrageous rates.
From this article on the economy of the Phoenicians:
From the interior they [the Phoenicians] obtained salt, which was highly prized in ancient times, the exchange rate being equal to gold. Roman soldiers (and probably Carthaginians too) were paid in part in salt, from which comes the old saying "worth your salt". Carthage had excellent relations with the warlike Gauls, Celts, and Celtiberians, from whom they obtained amber, tin, silver, and furs.
I find it fascinating that the modern English word "salary" (pay to a worker) derives the Latin "salarium", itself deriving from the Latin "salis" for salt. Whether Roman soldiers were actually paid in salt sometimes (quite debatable), they often would have bought salt with their pay, given its utility and valuable nature. Either way, this notable etymology, along with a number of common modern quotes in English, alone provide good support for the precious role salt once played in civilisation.
Some interesting facts about the role salt as played in human history (including quotes).
I don't think that gold and salt ever were equal in value, that's an exaggeration. Salt was very valuable however, particularly because of its use for conservation - valuable enough to make one very rich. This allowed cities that sold salt (e.g. Lüneburg) to get very wealthy and influential. The Wieliczka salt mine supposedly was responsible for one third of the Polish state income between 14th and 16th century. But they obviously had to sell salt in large quantities for that.
I have a great interest in the history of salt, mainly because of looking to the uncertain future, and can confirm that Salt was essential to soldiers during warfare. If was the main way to preserve meat, fish and vegetables, and without it long marches were untenable.
Salt mines were few and far between and taking it from the sea was only practical in certain locations. The weather / climate / seashore playing a large part in successful extraction, as well as operator expertise. See "Salt: A World History" by Mark Kurlansky.
Gold on the other hand is a metal that was found in reasonably pure form in some areas and rivers originating in those areas. It many have been in small quantities but it is very dense, so weight for weight it could have been the same price as salt before 200 BC.
Some interesting surfing did bring up a good estimate of the price in 200 AD; Eight grams of Gold would buy 2268 grams of Salt (about 5 pounds).
Eight grams of Gold is currently worth about US$496 which makes Salt rather cheap now.
Salt was much in demand, it is true; but is it very difficult to produce? I have a friend who used to make it on the beach in Sestri Levante in WWII, as a teenager and then walk inland selling it. It's not rare, just much in demand - unlike gold, which is scarce. If the price was that of gold ,surely more people would boil up seawater on the beach [unless they had the luxury of salt-pans].