Actually, the reason iron is so cheap is because life chemistry and tectonic-hydrothermal processes have concentrated it. The bulk of the reserves date back from the Great Oxygenation event from 2-1 bn years ago when the first photosynthetic life oxidized the iron dissolved in the ocean and precipitated it at the bottom of the then-ocean. This didn't happen to aluminium, which is more abundant than iron, to the same extent. Aluminium was only discovered in the 19th century and only produced economically as a purified metal in the latter part of that century. Iron itself took some technology to become cheap and abundant and before the 12th century BC was regarded as a precious metal (see: https://en.wikipedia.org/wiki/Meteoric_iron ).
The price of metals does not match their abundance in the Earth's crust and least of which the Earth as a planet.
Aluminum's also bound much more strongly to its oxides. Generally you need electrical currents to smelt aluminum, which wasn't possible until the 19th century (with one possible exception, below).
Iron, on the other hand, can be heat smelted, using charcoal or coal. Iron smelting drove the use of coal as it was consuming so much by way of forests earlier. Generally, fuelwood (locally available) is more convenient than coal, only available from specific areas and mines, and which requires transport over large distances.
Overland transport prior to the 19th century was something avoided if at all possible, so coaling routes generally involved ships and oceans or rivers.
There's some evidence that a goldsmith in Rome may have found a process for smelting aluminum:
"One day a goldsmith in Rome was allowed to show the Emperor Tiberius a dinner plate of a new metal. The plate was very light, and almost as bright as silver. The goldsmith told the Emperor that he had made the metal from plain clay."
From:
http://www.world-aluminium.org/history/antiquity.html
Most versions have him being executed by the emperor to prevent the devaluation of copper and gold.
The price of metals does not match their abundance in the Earth's crust and least of which the Earth as a planet.