There are three factors that drive the price of gasoline at the pump: The price of oil, the amount of taxes per gallon, and the distance from refineries.
As you'd expect, Texas and Oklahoma have lower gas taxes (by as much as 50%) and are much closer to refineries, driving down the cost of transportation.
What I find interesting -- though everyone likes to complain about the high price of gasoline at the pump -- is that gas is not that expensive, if you factor in inflation. In 1968, while I was in high school, I could buy gas for 30 cents a gallon. If you factor in inflation, that is $2.21 per gallon in today's dollars. See Inflation Calculator.
Some more information on gas prices and energy markets:
IEA Cuts Crude Demand Growth Forecast as Supply Continues Outpacing Demand
Pump Prices are a Treat for Majority of Motorists