Wrong hill my friend.
Alum as a dye was used extensively for hundreds of years, so it's not likely it was worth more than gold. I'll go out on a limb and just declare, it wasn't.
But to the point of aluminiums value compared to gold, the wiki tells us:
That's about 300 years too late for being a middle ages thing.Discovery of this metal was announced in 1825 by Danish physicist Hans Christian Ørsted, whose work was extended by German chemist Friedrich Wöhler.
Aluminium was difficult to refine and thus uncommon in actual usage. Soon after its discovery, the price of aluminium exceeded that of gold. It was reduced only after the initiation of the first industrial production by French chemist Henri Étienne Sainte-Claire Deville in 1856. In 1878, metallurgist James Fern Webster was producing 100 pounds of pure aluminium every week at his Solihull Lodge factory in Warwickshire, England, using a chemical process.