So costs do not factor into prices?
Over the long term competition will drive prices to a small margin above costs, but costs themselves do not set prices. In the case of Katrina the supply line was in havoc. Stations were trying scrambling to find the market clearing price for gasoline in the new reality of lower supplies. This pushed the price well beyond costs. Even then, under the threat of legal action for 'gouging' stations set prices too low and supplies of gasoline ran out before they could be refilled. I saw it happen in Tallahassee, and I know it was happening towns in the rest of the panhandle. The retailers had to find a price that prevented demand from clearing out their supply faster than it could be replaced. Sonny decided to collect a few cents less in taxes does not alter the price necessary to balance supply and demand, but it would reduce costs for retailers, and thereby increase their profits.
I'm all for tax cuts, and getting the government out of the road business, but I won't advocate tax cuts on false premises.
In a real sense that could have happened but the public perception was that the amount they payed at the pump decreased by 15 cents because a Republican did a good thing. I don't mind the retailers being the true beneficiaries of the cut.