Prices for gasoline are volatile because of the ever shifting price for crude, which still makes up the biggest share of the overall gas price (45 percent, 2016 average). However, prices also vary across the United States for other reasons. Often, greater costs are down to greater expenses for refinement, transportation and distribution. Also, gas taxes can vary by state.
As the infographic below shows, Hawaii out in the Pacific tops the list of the high-cost states, as it generally has a high cost of living, high gas taxes and high transportation costs. It's cheapest to fill up your tank in South Carolina, according to GasBuddy. On average the West seems to be more expensive, while the South is less expensive.
Alaska is interesting because it has one of the lowest gas taxes per gallon (8 cents) in the U.S. but one of the highest consumer prices for gas (2.8 dollars), which has to do with transportation and lacking refinement capacities. Though Alaska produces oil itself, most refined fuel comes from facilities further South on the Western Seaboard.
You will find more statistics at Statista