Summer grade gasoline costs more to make and refine because summer grade gasoline is made so that the summer grade gasoline does not evaporate as easily during the summer months.
When it's hot out the summertime heat can cause gasoline to evaporate from gas tanks, gas storage tanks, gasoline refineries and the gas storage tanks at gasoline stations.
So the gas refiners have to make it so that the gasoline is less prone to evaporate which causes the gas to cost more in the summer.
In the winter months the gasoline does not cost as much because it's not gonna evaporate as easily since it's cold outside.
Although in some places gasoline may contain an antifreeze additive to help prevent the gasoline from freezing up or getting moisture into it.