I don’t see your logic.
G100UL takes the same amount of alkalyte as 100LL within small margins, so what does the price of crude have to do with it?
As for other chemicals, it seems like the xylene component should actually be cheaper. Shipping and distribution should be less or the same. And avgas is not made in sufficient quantities to be sensitive to scale effects.
So the $0.85/gal premium seems to be a reasonable delta to remain.