It is counterintuitive that the addition of an ingredient (lead to gasoline) would produce a lower priced product. However, unleaded gasoline requires more refinement in order to be usable by internal combustion engines. Adding lead allowed oil refiners to short cut the refinement process. Fewer steps in the refinement process results in a lower cost product.
As with most material products, the cost of processing/manufacturing/producing/refining/distilling comprises a greater percentage of production costs than the cost of raw materials.