In the table you can see that the purchase cost of the raw material 'Aluminum " is higher then steel and galvanised steel. But because of the light specific weight and the specific caracteristics of aluminum it is much cheaper to process, handle and install then steel.
Aluminum will not require a protective treatment against rust so there again less costs.
And finally, Aluminum is a material that can be recycled for 100% with only a fraction of the energy needed to recycle steel.
No doubt: Aluminum is in the long run cheaper then steel.