Well, copper was useful because it was a significant part of making bronze and brass. Copper plus tin produces bronze, copper and zinc produces brass. Both hold an edge much, much better than any of the softer metals (but brass is more brittle).
Iron was even better at holding an edge and could withstand a lot of other abuse as well, which made it particularly good for things like plowshare blades. The problem which delayed iron development was that it required such considerably higher furnance temperatures to separate the metal from the ore. The upside was that once it could be separated, it was relatively available, a problem which had bedeviled bronze because of the relative unavailability of tin in the ancient world. Iron was widely available. And yes, iron was considered a very valuable commodity soon after it appeared, because of the initial relative costs in processing it. Once its ubiquity was assured by ready processing, iron quickly replaced most bronze tools, including particularly weapons. Adding carbon in the metal production process created steels. As an aside, aluminium was a high-value novelty metal until the ready availability of large amounts of electrical energy generated by hydroelectric dams to process the bauxite ore in to metal.
Iron was not particularly common (by modern standards) until modern blast furnaces were developed in the 1700s. In the 'Iron Age', Iron was valuable enough to be used as money - trade between the various tribes on the periphery of the Roman Republic and the Republic itself often consisted of the exchange of goods desired by the Romans for iron ingots (or scrap), at least until the Roman Empire was established and iron was deemed to be an imperial military good, which should not be traded with barbarians.
At the height of the Roman Empire, the total Empire-wide production of iron per annum was around 80,000t - comparable to the amount of iron in just two large modern ships, and rather less than half the amount used in the construction of the World Trade Center. Charcoal fired bloomeries, as used by the Romans and the pre-industrial Europeans from the end of the Roman age until the start of the Industrial revolution, just don't lend themselves to the kind of mass production that blast furnaces allow.
Iron replaced bronze for tools and weapons
despite being expensive, not because it became cheap. Arguably, the high cost of iron and steel (until very recently) was a major driver of the medieval social order, in which there were three major classes - the commoners, who were responsible for feeding everyone; the clergy, who were responsible for the spiritual welfare of everyone; and the nobility who were responsible for defending everyone. The latter task fell to the nobility, because nobody else could afford arms and armor, at least until well after this division of responsibilities had become thoroughly entrenched; And it would take until the French Revolution (and the outside France, the Great War of 1914-18) to break this pattern in Europe.
