The prices of all consumer goods can change over time. But when it comes to gasoline, area sellers say, people are always watching closely for the changes.

"Gas is a commodity, and people watch that," said Emmett Thomsen, general manager of Prairie Pride, which owns Cenex Ampride locations in Marshall.

So what makes gasoline more expensive in one area or city than another? It's a question without an easy answer, area gas retailers said Monday. There's a host of factors involved, from oil prices and fuel transportation costs to competition among gas stations.

Average gas prices nationwide fell slightly last week, according to data from the U.S. Energy Information Administration. For the week of Dec. 7, the national average price of regular gas was reported at about $2.64 per gallon, including tax, and about $2.52 in Minnesota. Prices around Marshall on Monday were slightly higher, around $2.59.

"There is no special formula," Thomsen said, and prices often present a moving target for area gas retailers.

It starts out with fluctuations in petroleum prices, retailers said.

"I think people don't realize the volatility of the gas market most of the time," Thomsen said. The price per gallon can rise or fall several cents in a single day.

Tara Finnegan, manager-in-training at the Freedom gas station in Marshall, said that while she couldn't comment on how the company sets its prices, gas prices in general are affected by petroleum markets.

"A lot depends on what happens in the stock market, and the price of oil per barrel," Finnegan said. "We can't really anticipate what the price is going to do."

Supply questions can play a part in local gas prices as well, said Thomsen and Jim Nuese of Kirkvold Oil in Hendricks. For example, a Twin Cities gas station might be able to offer lower prices because it's being supplied by one of two small oil refineries in its region.

"You don't have to go through a middleman," Nuese said, while retailers in southwest Minnesota have to rely on gas piped or trucked in.

"If it's piped to other places, it's more costly," Thomsen said.

Comparing Minnesota gas prices to those in South Dakota isn't fair either, retailers said, because South Dakota doesn't have a state gas tax.

Once gasoline is bought and delivered, retailers said the price at the pump still depends on multiple factors. Prices have to balance market changes, profit margins and competition.

Retailers usually try to keep their prices comparable to those of their closest competitors.

In Marshall, Thomsen said, "Realistically, as soon as one person moves (in price), the rest will move with them. And it's not always a specific business that goes first."

Tim Borstad, energy manager at the Farmers Co-Op Association in Canby, said FCA tends to consider competition spread out along Minnesota Highway 68, because that's where the customers travel.

"Minneota and Ghent look at Marshall," Borstad said, while the Canby location has to compete with other gas stations in Canby and neighbors to the north. "And then up north, you have what they call the 212 strip" where gas stations along the Highway 212 corridor tend to influence each other's prices.

While gas retailers need to stay competitive to survive, sometimes prices won't all fall at the same time.

"The worst thing is to go down in price, and then have to go back up," Borstad said. If the gas market is fairly stable, as it is now, he said, gas prices won't change drastically.